I'm really getting confused over some rules - please help ( in simple english

)
Open Show
Golden Retriever Puppy of 6 months and one week
Enter Breed PUPPY class (no minor class) - not placed first
Enter AV gundog PUPPY class (no minor class) - not placed first
Enter AV MINOR PUPPY - win class
supposing the above happened - would the pup be eligible for best puppy in show ? I presume not because beaten in other classes ? I understand about withdrawing from other classes if for example got first in breed puppy class and so on.
I just can't get my head round what'd happen if won AV Minor Puppy as maybe the dogs who beat you in the other 2 classes are 11 months and therefore not in minor ?
As you may have guessed I'm trying to decide what classes to enter at our first open show. It would be simplest to just win the breed puppy class :D and then I'd know what I was doing (withdraw from other classes and compete for BPIS) but I realise this may not be the case

Do the 3 classes I list seem sensible to enter bearing in mind it is Gundog day at Leeds so entries may not be too large at this open show ? Is 3 a sensible amount of classes for a pups first show ? Also if it makes any difference the Judge for the above classes and BPIS is a Golden Retriever breeder.
We're just looking to enter for the experience but I may get a nice surprise so want to understand the rules.
Thanks in anticipation of someone helping me get my head round it all :D
Sara (and Phoebie)
edited - it says NOT judged on group system so I presume that means no best in group etc which would confuse me even more LOL :D ???
sorry edited again - do winners of all classes including puppy classes go in BIS judging and then BPIS is after ??