am i going to overstock??

please, for the sake of your other fish, remove the betta. He'd chase your neons around until they die with stress. It seems to be a great myth on this site that bettas can do well in small community tanks. My lfs said no, I still tried in the 10g and quickly removed the betta. They simply dont' play nice.
 
Female bettas are fairly calm. I had one in a 10 gallon tank with some cories and a Black Tetra for years. I think she was almost as pretty as a male and not so territorial.
 
Fish and small shrimp schould be fine together. Make sure that the shrimp are well fed and have lot's of hiding places so they will not be hungry for or threatened by the other fish.

4 corys and 2 shrimp is more bottem dwelling fish then you really need in a 15 g tank. Unless you are constantly overfeeding 1-2 corys and the shrimp will be fine.
 
I have a betta male and female in a heavily planted tank with two cories and three swordtails and swordtail half - grown fry. All is peaceful and fine! the bettas hang out in the top of tank and are very peaceful with all!
 
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