With guppies, here's my recommendation, and I assure you that I received this or similar advice when I set up my 21 G late in the summer, and I may or may not have followed it (Check the sig)
Get 1 m, 2 f......only. That way, you can still put in a bristlenose pleco or a few pygmy cories or something like that if you wish. Here's the advice I may not have followed: plan for fry. Don't stock too much more unless you have a definitive plan for the fry.
I'd get the prettiest male you can find. I got a yellow jaguar myself, and I purposefully did get a slightly smaller one to assure myself he'd be young enough to want to breed (At another LFS later, I saw a lyretail version of the yellow jaguar and was sorely tempted, but I didn't want the two additional females, too.) Then, I got the nicest looking female at that store. (Well, I got two, but one died just after giving birth a couple of weeks later. I replaced her with another female from a different store so that the male wouldn't harrass the female I had too much.) Then, go to a new store and get their nicest looking female. This way, when you get fry, you'll get varied and interesting genes. And those females you buy will invariably be pregnant already when they come home.
I've had fish for 2 months now, and from the three guppies (with one female swapped out), I've had three rounds of babies, and the oldest of them are really showing their sexes and colors now. I'm going to have to give up all but a couple, and it's kind of sad, given how attractive they already are. I have quite a bit of genetic variety in them, too, as the male in the tank is the father for only one of the three rounds (and even that isn't guaranteed--livebearer females can store sperm for quite a while).
If you have other tanks in which you can add the fry as feeder fish, I would get a second trio (1m/2f) and really be very judicious about a bottom feeder option. Guppies are small, but the fry will inevitably up their bioload.
With guppies only, I think MTSs, an apple snail, and two kinds of shrimp would really be quite a nice little boon for the interest of the tank without upping bioload terribly.