yes you are over stocking.
no you shouldn't use chemicals.
no fish should never be seen as the clean up crew. they are ultimately the providers of the nutrients your algae is thriving on...
general rule of thumb. 1in of fish / gallon of water.
plecos get very big... some to around a foot
corys should be in a school of 4+
neon tetras... the same...
I suggest you take back all the cat fishes to the LFS and go for two ottos. they will clean the algae for you and have a smaller bioload.
I dont know how well planted your tank is, but I really dont recommend more than the Betta in there...