For a tank that size I would start with 6, as long as the shop you got them from understands that in the near future you will be returning some potential breeder size angels. This will bring your chances of a pair beyond 90%. You have plenty of room in that tank for a pair, breeders keep standard fin pairs in 20 highs, veils or supers in 29's. You need 5 gallons per potential breeder, I put 6 angels in a 29 on a regular basis to get pairs, then remove the pair to their own 20 or 29, depending on the fish.
I agree that the guppys may be a problem, but the angels will not be large enough to eat the guppys for about a year. Don't expect to see any fry.
I like the red eyes as well, this is one of the things I aim for when breeding silvers, along with a dark orange to the crown & a bit of spangling to the body. Traditional wild looking silvers are something more people should breed, they seem to have better irridescence, as well as general health, than many of the other color varieties.