If you were to only put full size guppies, I'd try some rainbows, like the bosemani or western (not the cheap 1.99 "australian rainbows" ). Simply because the colors will compliment and it will be a very colorful tank. He might even be able to see the dwarf rainbows because their body is larger (rounder) then most fish. The only problem I see with this mixture is that rainbows are very aggressive feeders and can be a little nippy at times. But, they are colorful and active. However, they don't go out of the way to nip others unless it's a rainbow.
basically, as someone else said, big fish eat little fish, you can't overfeed the big fish to get around this, simply because fish prefer live food.
you could also try rosy barbs (i've got neon tetra w/those), but stay away from young tiger barbs/green barbs/albino barbs (older ones would probably ignore them) My real suggestion, would be to nix the guppies in a 55gal, and do just the rainbows or other larger fish for your father in the larger tank. (but then again I'm not a big guppy fan LOL)