I've never encountered a poly that refuses tilapia filet or earth worms. If you're having problems I'd highly recommend those. Of the 4 polys I've owned none have ever touched blood worms, frozen or freeze dried, but each is individual and has preferences. For smaller polys I've had good luck with frozen beef heart, it gets a bit messy when they get larger and don't want to bother with the small pieces. But, as I said none I've ever met have passed on tilapia, and you can pretty much cut it to whatever size you need.
You'll probably be able to keep them in there for a couple months, it mostly depends on their growth rate, which is very much reliant on their diet.
