I'd upgrade the lighting if you can. I don't expect Dwarf Hairgrass to grow under 2wpg in a small tank, but maybe I'm just not very optimistic.
If you want it to have a chance of spreading faster, you'll want to plant it in very small groups (maybe 2-4 blades each).
That tank is a bit small for anymore fish. Otos need groups and I wouldn't keep 3 in that size tank. Maybe try some shrimp (ghost shrimp first) and see if they'll live. Otherwise, I think you are stuck with just the betta. 5g tanks are small, especially hex tanks.
As far as plants go, 2wpg is low for a small tank (they need more light), so you might be limited. Some Rotalas, like Rotala rotundifolia might be fine. Whatever you get, get small leaved plants. Some Anubias nana 'petite' would be good too.