The revert back into females without company :P

If you don't want a pair, I'd consider something else. Perhaps the Kole?

Do you think I can have a pair of Watanabei Angelfish in my 5g system? at least for couple of years until grow and then replace? I really like the color and behavior of this angel fish.

Do some research on anthias school before you get them. Long term, there success rate is very poor.