Yes 8 is fine. It's generally recommended as 4 minimum, so 8 will be just fine. There will most likely be scraps as they sort out the pecking order though. Have some diluted melafix (never full dose for bettas) on hand to help heal torn fins, and make sure the tank is very thickly planted so everyone can hide and have their own little spot in the tank
That's fine, dude. What size tank are you putting them in?