If you are going with the slim bodied neons,or cardinals,you could easily have a school of 12 to 15 in a 10 gallon.They do better in larger schools,and bio load won't be an issue as they are such small fish.I would go with cardinals myself.They are much hardier than neons,and do not get the dreaded NTD( Neon Tetra
As far as tank mates,if you fill the tank with a single species of tetra,which is your best bet IMO,this will limit you.Corydora,like tetra, need to me in schools of at least 6 as well,so this will over stock your tank.
A Betta might work,but they are all different,and you might get one that does not like to have tank mates in his environment.I have a 10 gallon that has around 12 glow light tetra that I keep 2 lyre tail guppies in as well.The guppies are always swimming around,and they get the terta moving.Before I added the guppies,the terta just kind of hung out in one corner of the tank.The guppies get in and stir up the school,and get the tetra out and about.Altough it is a small tank,it is one of my favorites because it is heavily planted,and has a lot of fish,with a lot of activity.
As far as the pleco goes,there are a huge variety of species that only grow to 3 to 5 inches.Clown,rubber lip,and bristle nose plecos to name some of the more common ones.Even if you have one of the smaller varieties,10 gallons is too small if you want to keep other fish.Plecos proudce a large amount of waste,and if you keep other fish with it,you will certainly have water quality issues.
So I would re-home your pleco,get a nice school of cardinals,and find some small,active,non schooling fish,to keep the tank interesting.