TL;DR - As long as you sufficiently explain and relate the power levels of all your magic users, you'll be fine.
To keep them just as important as the other users is going to be difficult, and it kind of comes down to how hard we hand wave physics.
The problem as I see it is that plasma is an incredibly high energy state of matter. To turn additional matter into plasma would require the input of significant quantities of energy. If you make this too easy for him to do, he becomes unstoppable. If you make it too hard for him to do, he doesn't stack up to the other users. This balancing is not something I can necessarily tell you how to do exactly, but I can guess that it is going to depend largely on installing a sense of how much skill these people have.
First, let's assume that Mr. Plasma's generation method consists primarily of pulling electrons out of local matter to create his source of plasma, and that how much he can store and control depends on how good he got. Day one, he might be able to ionize someone enough to guarantee that person gets shocked on the next doorknob they touch. Some time into his studies, with extraordinary focus, he could ignite a piece of paper at six feet with what effectively amounts to a small particle beam (but not control the subsequent fire). More study and practice yields the ability to create and hold a small light source. After two decades of constant use, he could have the ability to compress a large quantity of ionized matter into a small point, push it through the wall of a building, and release it, causing wild electrical transients (if electricity is a thing), chain lightning and fire.
Also note that Mr. Plasma, after many years of study and practice, could actually wield something that looks suspiciously like a lightsaber. Major homage possibility here.
As long as you take some time to develop each of the characters, and remain consistent in the way you relate power levels to each other, this is all totally workable, but will rely very heavily on your execution.