Keeping fish outside their comfort zone won't be instantly detrimental, it is a long term health consideration, and tank bred fish can sometimes adapt to wider ranges then their wild caught counter parts. A Betta might do fine in a pH of 7.2-7.4 with medium hard water. Upper extreme for the Betta, lower extreme for the live bearers.
It's always difficult to know if the fish we get in a pet store is tank bred or wild caught, and the employees are not likely to know as they just buy all their fish from a distributor.
Generally it is advisable to not mix acidic and basic fish together, but if your pH is in the low 7's and you don't have excessively hard water it can be workable. Live bearers really do prefer hard water though.
If you haven't already I suggest reading the fish profiles here, blue bar at the top, second link. Lots of good information in there for several species. Tropical Fish Profiles