Guppies reproduce like the spawn of satin lol. They're very pretty, and generally very hardy too.
I personally love mollies. I had 6 of them in my 10 gallon when I first started off (very overstocked, I know, but they lived and reproduced, so I obviously did something right).
They are generally rather hardy. Make sure you ask if they're kept in salty water when you buy them at the LFS. Also, buy them when they're as small as possible. Some LFS sell mollies that are already about 2.5-3 in., and those ones are obviously older and not going to adapt well to a new aquarium.
As with every fish, and every animal rather, they adapt to their environment. I'm not saying go out and buy 10 mollies and jam them into your tank, but if you want one or two, go ahead and buy them while they're smaller, and make sure you keep the tank CLEAN! Mollies can be rather messy, especially if you're not providing them enough food, they will seriously ravage through your tank looking for spare bits of algae to eat.
As for barbs, I am not a huge fan. The ones I have had before were always rather mean and picked on any other fish I had in the tanks with them, even the fish that were larger.
But if you're determined to have some mollies, lemme know if you have any more questions.