I have a black orchid (I think) male in a 6g with a few Endler's Livebearers. He's been amazingly tolerant of them. The orange/green/black looks nice with the betta's coloring.
i will probably get more than a 10 litre, but im not sure yet. i want a really good looking betta for it though so im looking out for a good breeder. tbh most males i think are peaceful unless another male comes on their turf . anyway a black orchid you say? nice! post up a picture of that beauty.
If you are getting a larger tank for the females, you might condier dividing the tank you have now for your current male and the new one. Just have lots of plants on both sides of the divider to keep the stress down.