Ive had a fish tank for over 15 years, so i might be able to answer some of them.
Im sorry to say it seems that you are already well over the fish limit of the tank. The 10 neons are ok, but with that pleco(Do you mean Plecostomus?) its to many. Ive had my tank taken over by one. He may eat the algae and crap, but he sure secretes waste alot. I finally had to turn the 14-inch baby in to the local fish store. ID wait till some die to get more fish or get a smaller Plecostomus.
Yes you do have to be carefully as to what fish you add as some seem to love little tetras. If you stick with more tetras of a different kind you should be ok. I don't think they eat or attack there own species.
-Do I need a CO2 system? (if so, will it affect the PH of the tank?)
Never got into plants that much to know what that is.
The light should be enough from your window, but you can get special florescence bulbs that replicate natural sunlight. Though you dont want too much light as your tank will be overcome with algae. If the plants look healthy id just stick with the light form your window.
"Is gravel enough for a substrate"?
Im not really sure what the hell a substrate is, but the plants i bought never really needed anything but a good anchor to keep them at the bottom. I gave up on plants because my Plecostomus kept devouring them.
Yeah when i had plants i had trouble with it blocking the filter suction, but thats the bad side of having plants. If you remove the filter your water will get polluted by ammonia and harmful amount of waste in the water. Either put up with filter maintenance or get rid of the plants.
Never really had shrimp so i don't know about this one. Just call the local fish store, they should know.
Ive had snails a few times, with little success. I didnt have trouble with them multiplying as the Plecostomus seem to eat the babies. They just never really lasted that long. I think their real PH or hard water picky.