My understanding is that ghost shrimp (unlike the red cherry shrimp and other types) don't breed in a purely freshwater environment - and anyway, your girls would likely eat any baby shrimp all up.
I've had one ghost shrimp in a tank, up to about 7. And my largest tank is 5 gallons - shrimps have a very low bioload impact. You could likely have at least 20 in a tank that size. (that's a guess, but I think it's probably a good guess)
And yes, some bettas will hunt the shrimp, but most of mine seem to leave them completely alone. Even the smaller ones. Mind you, they ate all the yellow shrimp I tried to have.
They are copper sensitive, but stress-coat doesn't bother them, at least mine don't seem to care about my water conditioners.
I have heard from multiple sources, and have personally experienced, that they aren't the most long lived creatures around. But they're entertaining.
A Fish Called Jayne - White Marble Half Moon
Coco - Chocolate Veil Tail
Gimli - Teeny Red Crown Tail
The Bursar - White and Red, Butterfly Half Moon