Full size crayfish will absolutely destroy a betta. Dwarf crays, however, are safe to be kept with bettas. Just make sure you provide some small caves that the betta can't get into or it could easily eat them when they molt. How many dwarf crays you could keep is a function of how large your tank is.
What kind of feeder fish are you talking about? Most feeders I have seen used here are either low grade guppies/livebearers, rosy barbs (I think it's them anyway) or small goldfish.
Pet stores do a lot of things that a considerate fish owner wouldn't and shouldn't.
The ones they have are feeder goldfish they also have rosy reds. This petstore by me is the worst of any petstore ive ever been in they been introuble at least 3 times. Im not sure if its normal but when they were bagging the feeder fish they spoon the bag so fast seem that spinning them is unsafe and they said they were stinking extras incase they die. Next time I go back I should take a vid. Went to walmart not long ago and they had dead fish in their tanks.