I dont see why they aren't good to mix. Are the lemons about the same size as neons, and are they aggressive?
If the answers were yes, then no, they're fine together...
As for schooling...
At first when they are new to the tank, they will use shoaling as a "comforting" method, and will most likely stick with its own kind. After a few days/weeks, they should become comfy with the new home and each other and the shoals will break up and they will intermingle with one another...