Guppies, depends on if you get feeder guppies or fancy guppies. Feeders only get about an inch long. They are very plain and really kind of ordinary. Fancy guppies get up to about 1.5-2 inches or so and have much more brilliant colors, especially if you get a male that is different colored than the female. The cross colors can be quite intriguing. They do tolorate salt well and I use stress coat. Since I have set my tank up 8 months ago, I have never had a disease in my tanks.
One thing to make sure of is that the tank is well cycled before investing in fancy guppies. They are not as easy to care for as feeders. If you need to find out how, go to the general discusion, it has some really good information. Use feeders to get the tank set up if you use fish. When it has set up for about 6 weeks or so, then the fancy guppies will do a lot better.
Take all the feeders out of the tank and exchange them for fancies if you can or give them back to the LFS
The fancy guppies are very active and will eat like there is no tomorrow. Don't overfeed them as this can be detrimental to their health. Make sure the ratio is no higher than 3:1 female to male. I would say 7 females and 2 males would be fine but watch them to make the males don't fight each other.
If you give them a good diet of flakes, shrimp pellets, blood worms and other good foods you should have real good luck.
Don't be discouraged if the females die after their first mating. This happens more often than not, even with swords. The only way i got my swords to live after their first brood was born was to not have male in the tank until the females were looking like they were comfortable and healthy. Then I added 2 males, one died and the females are now working on their 6th brood each in less than 6 months.
Be patient and they will breed with ease. Give them lots of floating plants and the fry will survive in the same tank. If not plants, take the female and put her in the breeder tank then transplant the fry to breeder tank. Do your water changes, follow the basic instructions, I use the recommended amount of salt from the box and I always use stress coat of some kind. Remeber to replace the correct amount of salt and stress coat after each water change.
You can tell when the female is about ready to give birth because the area before her tail will darken and you may be able to see the eyes of the babies.
Best of luck and if you really want to be adventurous, try swords, they are much more active, though maybe not as colorful.