Mollies and guppies can both live on their own, but if you go stick a few nice sized mollys into a 10g tank, they are going to be quite angry and probably do a fair bit of chasing and nipping. I have a molly fry tank that is 10g, and by the time they are 3/4", they are far too large to continue living in the tank. I would hold of on mollys until you have a bigger tank, you seem to like guppys, there are so many variations and kinds, it really is an entire branch of fishkeeping in its own, and guppys do fine in a species only tank.
What i suggested to you before I still believe to be an extremely neat and viable option. I personally have not kept one, but am planning a female betta tank and have seen some REALLY NEAT tanks. The cool thing about female bettas is they dont fight if you keep them in good numbers, a 10g would be able to house up to 10, but a good number would be 5 or 6 bettas, at least 4 so they can establish heirarchy. Also, female bettas are CHEAP CHEAP CHEAP at the stores, you find them in those cups for $1-$2 a piece, and you can make a rainbow of colors. This is a great suggestion for a newbie tank in my own opinion, and gives you a lot of options for different color choices, without leaving you so many options on fish species that you end up with a mismatched tank.
The same can be said with guppies, although you can keep males together, and probably wont find females much as they are not wanted in the guppy trade, not pretty enough. You can really mix and match colors to get just what you want.