I'm not sure if a RTC will like veggies, but anything that isn't acidic or irritating (no onions, tomatoes, etc) should be alright. Carrots, parsnips, zucchini, cucumber, and leafy greens are all good foods for various fish.
Crayfish are perfectly fine, and your cat will probably love them. I'd be just a bit cautious about feeding live crays to a smallish cat, though, it might pinch onto the whiskers. Frozen crays don't have that issue. Depending on the size of your cat, you may want to remove the carapace (thick shell) and claws in order to help prevent gut issues from shell blockage. Table shrimp are much easier to de-shell, so you might want to try them instead. Plus, you can buy table shrimp cheap. They aren't the best staple, though, since they're usually sold without the guts and are therefore lacking nutrients. Head-on table shrimp are a bit more trouble to prepare (they have a rostrum that should be removed), but they should have plenty of nutrients. Still best to vary the diet.
There's a Cabella's near us that has two massive tanks full of various non-native fish people have caught in lakes near us. Oscars, a couple of nonnative turtles, common plecos, pacu, a couple of shovelnose cats (very pretty), and three redtail cats. One of them is almost 5' long and as big around as my ribcage.