Not really sure, FBTs will eat anyting that moves and they can fit in their mouths. I wouldn't use guppies as their staple diet though, maybe as a treat every now and then.
Keep in mind FBTs get their coloration from bright foods such as carrots, oranges etc so it's easier to feed those to crickets (along with gut load for calcium and other vitamins etc). I'm not so sure that fish food will give them the same color through the guppys.
Lastly, I'm not sure how good FBTs are at hunting underwater prey such as fish. I know my guys will eat crickets that mistakenly jump in the water but those crickets are kind of helpless at that point so it's an unfair fight.
Hope this helps at least a bit
