
I've 5 dogs, at 5 1/2, 4, 3 1/2, and 2 x 2yrs. The latter 2 are litter brother/sister but I hasten to add NOT raised together, the boy I've only just acquired. I didn't set out to have 5 dogs this young, it's just the way it worked out. What I would strongly advise is to have a minimum of 18 months between dogs and more if there is still training to be done.
It makes sense to get a dog fully trained, and to also enjoy it to it's full before introducing another dog.
Enjoy your young 5 month pup whilst you can. It's not fair on him to bring in another dog requiring lots of time and you're going to miss so much with the JRTx. Waiting until he (or she) is older, means you are also much more equipped to cope with the next pup and it's training.
You've already got 2 very young dogs, enjoy them whilst you can ... wait a while before adding another. This is from one who knows!!