Unless your tank is less than two gallons. they'd be fine in there, same with your betta boy. It'd probably be less stressful for him to stay in his tank anyways. Bettas can actually go without eating for a really long time. After a week, he'd be hungry, but not hurt. A wafer would probably cloud up the water too much. Just try and gut-load them and then do a water change before you leave.
If you have a smaller tank (less than 5 gallons), you might want to enlist the help of a neighborhood petsitter for a partial water change while you're gone to keep the water clean (just give really good instructions), and maybe even have the person drop in a pellet or two for your betta.