You can keep several guppies in a 5gal. I used to keep guppies extensively. I'm in a situation where I can't have a tank now. I've had large tanks in the past, but in college, I set up 5gal hex tank with a filter/light built into the hood.
If you do regular maintenance, (ie: change 25% of the water once a week, clean the gravel once a month, change the filter at the time designated on the instructions, and condition tap water if that's what you're using) then you'll be just fine. I wouldn't put more than 4-6 guppies in it.
As far as food, I always feed a tropical fish flake to enhance color. They can nibble off bits of larger flakes and if you have babies, a distinct possibility with guppies, you can take an old, empty container ( I used an empty beta food container) and crush the flakes to a fine powder with your fingers. It makes a good baby food.
The other cool thing about guppies is that they breed easily. If you get into them, you can get a bigger tank and use the 5gal as a baby tank. I used to pick colors and fin styles I liked and then when they'd breed, I'd keep the babies I liked and give the others back to the pet store. If you keep that up and introduce new blood occasionally, you can design your own guppy. I ended up with USC Gamecocks guppies before I got done with it. They had black bodies and burgundy tails. :))