Hello,
To my knowledge, runny poo is NOT normal. My baby white's had (and still has) runny poo, I have been treating her for hook and pin worms for over a month. And a protozoan (bacterial) infection for 14 days in Feb and March.
I would highly recommend you get a stool sample to a vet for testing.
1. I have never fed my white's fruit flies, so not sure if it would affect their poo.... but, I have a feeling it would still be firm, if the frog(s) are healthy.
2. I don't know? I'm surprised they ate them, your frogs must be quite small? I would gander to think it would be okay.
3. small crickets are all I feed my white's. Don't forget to dust with a calcium powder with D3 at least twice per week, and a multi vitamin for amphibians once per week.

And just for comparison sake, my white's acts and always has acted completely normal, with no affect to her appetite due to the infection.