I think a 5 gallon may be too small to be adding anything else to your set up. Im no expert though so someone else on here will probably jump in.
I started with an 8 gallon for my Betta and I added around 10 red cherry shrimp, which i think are petty similar to keep as ghost shrimp. I upgraded my tank to 23 gal recently, basically because the shrimp numbers had tripled and i wanted some other stuff in their. I had no problems with them getting on though, the shrimp are rapid and almost teleport when the betta shows some interest. Though I think this depends how chilled your betta is.
Hope this is of some help anyway,