Feeders: Feeder Guppies tend to be smaller then Fancy Guppies, so at just over an inch long for males, three or so should be fine in a 5 gallon, but oviously, the more room, the better. Feeder Guppies are usually Endlers Livebears, a rather hard to find, little fish. Most of the time they are bred with Guppies, so you are getting a hybrid, rather then a pure Endler.
Fancies: These guys tend to top out at 2 inches for the males, but there can be exceptions. 10 gallons for a trio (1 male and 2 females) will be fine. I have even kept about 15 in a 10 gallon, but I did massive water changes every week, and was very careful not to over feed.
Guppies love to eat, no doubt about it. They will eat anything that floats in front of their face, even their own poop, so feeding them shouldn't be a problem. Mine really love Brine Srimp and Blood Worms (when they can get ones that are small enough to eat ) and will go for any kind of flake food I put in the tank. They even like Algae Waffers!
Since Guppies are pretty peacefull little things, you can pretty much have what ever you want with them, other then fin nippers. I had some Bloodfin Tetras that tore up my favorite male up so bad that he has permanent fin damage, but I will say that Bloodfins are very curious little things, and they like to be in the middle of everything. If you want to keep mixed sexed Guppies, it is best to have 1 male for atleast every two females. The males seem to not get along much, and are always squabling if there aren't enough girls to go around (Ha! Sounds like High School ), and they tend to rip up each others fins, even when kept in an all male group. They also like to try to mate with anything that moves, so make sure you get fish that dont mind getting pestered for a while, while the male Gups figure out that what ever it is isn't another Guppy XD. It seems to me that an all female, or a mixed sex group is the way to go, as it will be more peaceful.
Guppies really like to have plants to play in. Mine love to swim in and out of the plants and caves I have in the tank, and they really seem to like sleeping on wide, flat leaved plants, and any other semi-flat surface. If you have any males with long fins, make sure that they plants you have will not catch on their fins. Silk and live plants are always the best ways to go.
Female Guppies can store sperm in their bodies for 6 months and use to multiple times to get themselves pregnant. That said, females only have to mate one time, and they can continue to have fry for 6 months. Pregnancy lasts for about 29 days, and the fry are automaticaly concidered food by every other fish in the tank, besides the female dropping them (that is, untill she's done!). The best way to save the fry is to get a breeders net, breeders trap, or have an extra cycled tank set up to put the momma Guppy into while she is dropping. If you put her in too soon, she may stress out, and prolong the due date. New born fry will tank anything you give them, as long as it fits in their mouths. Crushed flakes are the easiest to feed, but some feed Baby Brine Shrimp, and other live foods. I find that fry grow the best on a high protien diet, so I give mine alot of BettaMin and Omega One Betta Buffet Flakes, along with Omega One Freshwater Flakes, and a few other things.
Feel free to add, dispute, whatever =)