Its on the small side for a mystery snail they need 10 gallons for initial adult snail so a nerite would be more appropriate. ADF males sometimes compete but no serious injury is occurred. If you get a male and a female their is an issue that they breed and lay eggs which the betta then eats and overloads his system over time from the very high protein diet. They are easy to sex just look for the dots behind the front arms some don't show it very clearly but most do and the dots signify males. I would say get 2 of the same sex but if you can't id then its no big deal.
ADF's will but normally they are too slow to cause any significant impact on population. The betta may take a disliking to shrimp and eat them but normally not.