I have got an 18 month old Golden Retriever Girl, and my husband and I are thinking of getting another dog at some stage. My girl is very good, obedient, very happy with other dogs, large or small, playful, but very laid back and not a super energy dog at all. in fact, she's starting to work as a therapy dog right now, as she's very friendly and calm.
I was wondering if you have any advice on when the best time for dog no 2 would be? Also, have you got any advice on a breed/sex? As I have 2 younger children (5 and6), it would be nice to go for a slightly smaller breed this time. but then I'm not sure which breed would be suitable for a laid back Golden? Would it be better to go for a puppy or a slightly older dog/bitch?
Any thoughts or advice would be greatly appreciated.
thank you!

Because your Goldie girl sounds like she has a fantastic temperament etc i wouldn't have thought it would be a problem introducing a puppy.  I know some would advise you to get a male but imo it depends more on the personality of each individual dog.  We have had a Goldie in the past, along with a 'show type' cocker spaniel...they were a fantastic match.  Both were full male's. Personally i would be looking to get another puppy/dog in the next few months, with such an obedient, friendly, calm dog such as yours.

I advise 3 years as the ideal age gap.

Something else to consider is how many dogs do you plan to have?

If it is two, then it is best to have them roughly half their lifespan apart.  This way your never in the position of having two old (possibly expensive) dogs together, that you could well lose one after another.

My friends Golden lived to 14 plus and that is not unusual, so I personally would wait until yours is about 6 years old to get a puppy.

If your considering an older dog then something like a 6 to 8 year old male from breed rescue whose history is known will do the same thing.

That way when the older of the two goes you will still not be dog-less, can grieve and then get another puppy.

As for breed, the canine world is your oyster.  I just wouldn't' go for anything that had widely different exercise needs.

I have a Golden Retriever bitch who has the same temprement as yours. We also have a male English Springer Spaniel, they get on very well although the Springer is far more lively and runs rings round the retriever. To be honest, with a breed like the retriever, the world is your oyster. They get along with everything and anyone so you can choose your favourite breed. I dare say some will say as both are gun dogs they would be most likely to get along.  

I say at least 18 months apart and that you need to be sure that you're first dog is settled with no issues etc, that may take time away from training a new dog.  If she's well trained then a 2nd dog shouldn't be a problem.  But I'd think about the time you'll have available for training ... you will need to put in quite a bit of time with a new dog, and how much time are you having to put into her therapy training?  Can you afford the time to give both the quality time/training they need? Or would you be better waiting til your girl is a therapy dog and then having another?
The other thing I would think about is:
if you have 2 young dogs together - you may well lose them at the other end of their lives within close proximity to each other
they will slow down together at the other end of the age scale
If you have a greater time span, then when the older dog goes (one would hope it would work out that way around), then you're left with a solo dog who may well pine and not be happy as a solo dog, so do you get another one?
I set out to have 2 dogs, 18 months apart.  I do a lot of mountaineering and I didn't want to have an older dog that couldn't cope with the exercise and a younger dog that could!  I now have 4.  First 2 are 19 months apart - ideal in my opinion and circumstances.  2 and 3 are 10 months apart - too small a gap.  It was hard to work around the training that they all do.  3 and 4 18 months apart.
My first girlie is a VERY strong minded bitch.  I decided to go for a boy for doggy number 2 and I'm glad I did.  He's just soooooo easy. In fact if I could have 10 just like him I would.  Numbers 3 and 4 are girls.  Everyone gets on very well - it never occurred to me that they wouldn't, which I think helps.  I guess if your Goldie Girl isn't spayed then that's a consideration as to the sex of doggy number 2.  It isn't a problem with me with unspayed bitches and an entire male in the household, but I know a lot of people who don't seem to be able to cope with the hassle around season time.
2 dogs - PERFECT!!  As to smaller breeds I've no idea ... don't do small
Hope this helps.

Edit: sorry if this seems a bit garbled, I know what I mean ... hope you manage to work it out!

Based on my own experience I'd plump for 2 years as the minimum gap and 3 years as an ideal.  You want your first dog to be well enough trained that it won't all go to pot when the pup comes!  You also want a dog that is secure enough with it's own position in the family not to mind you spending time with the new arrival, and that comes with some maturity.  Training is so much easier and more effective done one to one but you don't want your older dog to feel left out.

My eldest was actually 4 when our pup came and he was perfect.  Young enough to enjoy it, but very self confident and quite happy to take a back seat for a while.

When you are trying to decide what breed to add it is worth starting with your existing dog as a blueprint.  Have a think about what you like and what you might wish to avoid next time.  Mine was quite straightforward - I had a whiskery terrier type mongrel who is a bit of a barker and prone to chase livestock.  I love the wire whiskers but was happy to lose the barking and chasing.  I got a wirehaired vizsla.  He's perfect too!

Incidentally mine are both boys - the first is neutered, the second is entire.  They get on very well.

Your goldie sounds lovely,hope you dont find your next one not so easy,but if you get a puppy the older one should set a good example,which will help train the new comer.I would say do not put off getting the second one,a big age difference means two dogs needing different levels of exercise as the first one gets old,also the older dog would not enjoy a hyper active pup around.Summer is a good time for training a puppy out of doors so why wait?My local dog rescue will not place dogs with people who have young children,which I think is wise as their history is unknown.So many smaller breeds to choose from,but maybe a spaniel or if you want to go even smaller a terrier type would match your goldie for stamina.If you want a dog small enough for the children to pick up,nurse or hug,I would recommend another bitch,as to put it nicely they have clean tummies,whereas the dogs often get a bit damp and smelly when they aim at a post and miss!Also unless you want to risk goldie cross puppies I would not get a dog,keeping them apart while the bitch is in season,it not easy,when you have children who leave doors or gates open,and if you live in an average size house,the scent of the bitch is very upsetting for the boy.Best of luck which ever you choose.