As far as I know yabbies don't need to eat fish, or meat for that matter. Vegies like cooked carrot, pumpkin and spinach should be enough. The odd bit of fish wouldn't hurt, but I wouldn't bother with live fish, just chuck in a small chunk of the cheapest fish on offer at the supermarket (or even better, carp if you can get it). AT the moment I have a Yarra Crayfish in one of my tanks. It is living well on the bits of rotting plants (mostly java moss) and the odd small piece of vegie scrap.
BTW, yabbies will escape from even the best tank. I have been amazed by the ways they find to break out. Climbing up airlines and lifting the lid (perspex) off the tank seems to be their best skill.