Can a peach face lovebird gel alone with a fisher lovebird?

It is possible, it really depends on the bird itself. If you want a new lovebird, consider just introducing the birds first with no physical contact (i.e. two cages next to each other) and see if they get along. It is more likely for two males or a male and female to get along than two females.

It is possible for a Fischer's Lovebird and Peach-faced Lovebird to breed so if you don't want chicks, don't put a male and female together.