welcome to fish lore!!
as for your questions, number one depends on the size you get but shouldn't nip at each other, i have have bala sharks cramped in a bag for awhile no nipping, if you are worried though you could get one of the critter boxed with lids, they are still small but bigger than most
LFS bags.
number two: as far as i know it doesn't matter and "petstores" such as a variety of pets other than fish, generally can't,don't, or won't.
healthy barbs have good clear eyes, scales without blemishes no streaks of bright red or white dots, or malformed fins(as these are all symptoms of diseases).. also if you take the time to drive 3 hours for your barbs take atleast 10 minutes to look at the fish, are the swimming nicely together, are any panting, or restlessly swimming, signs of problems aswell..... when you get your fish remember to place in a
QT for awhile... best of luck!-Steed.