White Cloud Minnows are not a bad idea, they appreciate cool water like X. Laevis does. I'm not sure if they are 'top swimmers' but if they are, they would be advantageous to them because X. Laevis are mostly bottom dwellers. In a large enough system these fish may survive but like you said, it's only a matter of time. They have to rest or will just get careless and the frogs will clean house in due time.

I used to keep guppies and mollies with my frogs, they were all eaten. I think zebra danios may work well too because they are also top swimmers and pretty fast too. In a closed off system (as aquariums are) even in a very large tank, the advantage always goes to the predator.. so I wouldn't get attached to the fish and I wouldn't pay too much for them either.

If the fish are healthy and you have a bulletproof LFS or you breed your own and you're good at diagnosing fish illness and you quarantine you certainly could feed fish to clawed frogs. It's just that it's more work than it's worth and mistakes do happen. If you've had luck for 7 years feeding guppies I won't argue with that. There's always risks to live food but you can minimize it greatly by being smart.