A lawnmower gets to big for a 12g nano and no, I don't believe it could support one for very long. Lawnmowers prefer to eat the green film algae. I know people suggest them to control hair algae but mine never touched it. I did have a ten gallon tank though that was completely covered in hair and within a week, the mollies had it cleaned up. You don't need many. And acclimating mollies should be done over a period of a few days, constantly raising the salinity til you reach the salinity of your tank. My mollies were sold to me already acclimated and they are lyretails. If that makes a difference. I have an article on sw
mollies, if I can find it , I'll post it.