I would disagree on the pleco. Most plecos sold in fish shops - such as the common plec and gibbiceps plec - get much to ig (over a foot) for your average community tank. They also are very messy fish that produce a lot of waste and need extra feeding besides algae. Many also require wood in their diet.
I'd also say that relying on your LFS for advice is a bad idea. Research possible tankmates yourself. Asking here is a good way to start but also look in the fish index, on fishbase.org and do a google search.
When you say you 'cycled' the tank, what exactly do you mean? As you are new here (welcome BTW) there's no way of knowing who's advice it was you followed to 'cycle'. If you just let the tank sit without fish for a while like some fish stores tell you to, the tank is NOT cycled.
What size tank do you have? How many gallons? Also, do you know your pH and have you tested your ammonia, nitrIte and nitrAtes? You can use pH to help narrow down your fish choices (though most fish will adapt to your pH anyway) and you need to know the rest in order to be shure the tank is safe for your finned pets.
To answer the question in a little extra detail, guppies would work but they (as with mollies) are livebearers so may breed and multiply in your tank if the fry have enough places to hide. This can sometimes lead to over-stocking. You can avoid this by sticking to single-sex groups. In your case it's too late with the mollies. That brings me on to the next point about keeping livebearers - they should be kept at a ratio of 2 females for every male. The reason is that in this way the male's attentions (chasing) are devided so that no single female becomes stressed. The more females to every male the better obviously. The same applies to both mollies and guppies as well as wordtails and platies which were mentioned earlier as good tankmates.
As for tetras - most make good tankmates but, as they are schooling fish, should be kept in groups of 6 or more. Some (such as the ever-popular neon) need a mature tank (at least 6 months old) and should not be added to a newly set-up aquarium. Regardless, you should add fish slowly to avoid deadly ammonia spikes. Keep in mind that 'tetra' incoroporates a huge group of fish. Some are more suitable than others - ranging both in size and temperament as well as in requirements and hardiness.
Good luck with your fish
