The RTBS will outgrow a 2 foot tank, as they can easily grow to over a foot. I have personaly seen them at 13inches
The Common pleco will get to between 12 and 18 inches when full grown, and will grow real fast to 8 inches, so you either need to be purchasing an upgrade round about now, or finding a friend with a 4X2X2 foot tank minimum for it in the comming months.
Without to two fish above, you would have about 10 inches of adult fish. You have 13 galons of water (well 13.3333recuring

) so for the first 6 months, 13 inches of fish is you limit. Another molly or two would be good, particularly if you have 1-2 males ATM, as the lone or pair of females will be pestered by the male(s) to breed. The aditional female will take some stress away be spreading the burden on the other female(s).
After 6 months you may get away with a few more, but considering the other fish, I'd leave it at that.
My Priority in your situation would be to sort out the RTBS and pleco
HTH
Rabbut