My Oscars LOVED the guppies I fed them.. I had 2 of them and bred my own guppies for them. Even when they were both the size of a softball they loved the guppies I fed them..
I think it is a great idea.. A couple of pointers for your guppy breeding...
1) Divide the tank so the fry have a safe place to be. Or just hang those little breeder nets along the top.
2) Keep the
pH around 7.5 and the temp around 78 to 80... They seem to breed more and better that way..
3) If you have a
HOB filter keep it away from the fry... It'll suck them up like dust mites into a hoover.
4) Keep the heater away from the fry as well...
5) Give the fry a plant to hide in... They like that. I used to keep floating plants at the top so they were always at the top which made feeding them easier for me.
Other than that just try to keep the inbreeding to a minimum if you can... If they inbreed to much you may get midget fry and run out of food for your Oscar.
I hope that helps some.