I have a pink spotted watchman and a lawnmower and they are fine with each other. They were some of my first fish. The sea hare is great too, I have one too. BUT they can easily get sucked up by filtration and powerheads so they don't usually survive for too too long in a home aquarium. Plus after they eat all the algae you have to supplement their food with algae sheets.
I've had my sea hare for like 8 months and people are shocked I've managed to keep him this long. lol