Ah okay, well there are a few stocking issues that I want to point out--don't worry you aren't in trouble or nothing but I have a feeling you may have been given the wrong advice or didn't do some research?
Which kind of Pleco is it? Even the smallest of Pleco's will get around 4-6 inches which is very large for a 10 gallon, they also poop a TON! Anything that eats algae is a very messy fish and for a fish in a 10 gallon you would need to do nearly every other day water changes just to keep it clean. I suggest you find a new home for the little one or take it back to the store if you can. I know it sucks giving up fish but for the health of your tank and for your other fish as well as the Pleco, it would be a good thing to do.
Mollies can also get up around 4-5 inches depending so ideally you would not have them in a 10 gallon either. What is happening is that you are overrunning your biological filter with ammonia (fish poop and other waste) and because you most likely have high ammonia your fish are stressed out. Since you've been doing the water changes you are getting rid of the ammonia, allowing time for the good bacteria to catch up a little bit which is relieving the stress off your fish which makes them feel better. Make sense so far?
Do you know about the Nitrogen Cycle or Beneficial Bacteria often abbreviated BB?
In a 10 gallon it would be best if you just had the guppies rather than all that fish, also guppies and mollies can interbreed so just be careful of that; it can make some really deformed fishes sometimes which isn't something you really want to deal with I assume :-/
Here's a fantastic site that will help you stock your tank and figure out what your tank can hold if you do the proper weekly water changes, I use it all the time! AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor