Mentalism is not psionics, as explained in spell law:mentalism. Main difference is, than when psionics would las tonly as long as you concentrate (power of mind and concentration), mentalism is real manipulation of mana, just like essence, and continues to work even after you stop concentrating. Mentalism is manipulation of mana inside you, as opposed to essence, which is manipulation of mana around you.
Main differences between mentalism and essence are that essence is flashy, stuffs like illusions and fireballs, and mentalism is subtle, like mental communication and mind control. Mentalism also doesn't suffer from armor, only from helmets. But yea, there are common points in psionics and mentalism. And if you want to use RM magic, you can easily swap HARP-magic system to RM-magic system.
