You didn't say how many of each sex you were buying. I'm assuming both tanks have good filtration and ammonia levels are nil. You should be able to house 2-3 really nice males in the 5 gallon. Be sure to leave room for a couple of scavengers. Snails, small corys or otocinclus, or a few ghost shrimp. I encourage live plants. I use Anacharis almost exclusively for my guppies. It is an excellent oxygenator and its rich green color is complimentary to the colors of your fish. It also reduces stronger current so the male guppies don't wear themselves out maintaining their position in the tank.
Your ratio of males to females should be 1:2, if you intend on breeding them. Unless you want about a billion babies right away, do not put the males and females together. Observe your females closely for about a month, for signs of an existing pregnancy. Either plan on buying more tanks to put the fry in, or heavily plant the girl tank with anacharis, or other bushy plant. Bear in mind baby fish need a lot of room to develope properly.
Feed them well with quality food, including some live food. Adult brine shrimp are good. When you see qualities you like, size, color, fin length, etc, you can place one male with 2 females together until the girls become obviously pregnant. Return the male to the bachelor pad, and set up your nursery to your choosing.
Worked for me. Good Luck! Guppygranny
Yeah I know all that stuff. I had guppies for 6 years and I'm pretty experienced with them. The things that I'm worried is if the fancy males will fight each other and damage each other fins over the fancy female guppies. :-[