Any type of livebearer would go good with guppies. Platies, mollies, swordtails and a small school of cories. Look at my signature and see what I have with my guppies.
I think with guppies in that size tank the last thing you are going to want to do is to add more livebearers. Even if all the guppies are one sex, unless the other livebearers are the same sex as the guppies there could be a risk of cross breeding.
With guppies in that size tank, you could go for cories, as Sasha suggested, or perhaps a couple of otos. With cories, I would get smaller breeds as some can get very big such as bronze. Instead of cories, you could get kuhli loaches perhaps.
Just be sure to get a good mix of top/middle/bottom dwellers and try to not overload your tank.
As mentioned above, cories or ottos would be nice cleaners for a guppy tank. Depending on how many guppies you have, you may be able to add 4 small tetras(neons, glowlights, black neons, or cardinals) .
Some nice top fish are Hatchet fish and Glass cats. Both of these pretty much ignore every one else. I have also had Phantom Butterflies with guppies and they stick to themselves too.