It is possible the big one could decide to pick on the small one, you'll need to keep an eye on them for a while, hopefully the little one will grow and catch up with the big one.
I agree, the tank is too small for a red tail shark. It is too small for anything with the word shark in its name.
In your other thread, you said you've just got the API test kit. Have you tested for ammonia and nitrite yet?
I've not quite worked out if this is a brand new tank and you've just added your mollies as the first fish? If this is the case, you can't get any more fish until both ammonia and nitrite stay at zero. Until then you'll need to be doing large water changes every time you see a reading above zero for these two. They must not be allowed to get above 0.25 or the mollies will get poisoned. They may not die straight away, but they will be more likely to get sick and die young.
Once you've had zero ammonia and nitrite for a week without needing to do any water changes it will be safe to get more fish; very slowly. You'll only have grown enough bacteria in the filter to get rid of the ammonia made by the two mollies. If you get a lot more fish at once, the bacteria won't be able to cope until they've multiplied to make more.
Of course you might have done a fishless cycle using ammonia, in whcih case ignore that
The two mollies take up quite a bit of the the stocking allowance for your tank. Even the smaller black mollies take up a fifth while the bigger sailfin mollies take up a third, and are probably too big for the tank.
I wouldn't put a betta in a tank with mollies. They can be quite agressive and a betta with his long fins could get picked on.
As for what fish to get, you are a bit limited in choice. You need small fish to fit in your tank, but fish big enough to avoid being eaten by the mollies.
Do you know if your water is soft or hard? Does your shower or kettle get furred up - that's hard water. Your water company's website should also tell you somewhere on it. It is easier to choose fish if you know the hardness.