powered speakers have amp sbuilt into them. So from the receiver's point of view, the signal flow is into an amp, not a speaker, when going to the powered speakers. You need a receiver with line level outputs for the left and right channel in order to use those speakers.
You could connect the Motif into the receiver using quarter inch to rca cables (or a quarter-to-rca adapter on the receiver end, then play it through the receiver, just switching to whatever channel you used for the Motif connection. Might take a little experimentation to find the output level on the keyboard that works well with the input expetations of the receiver, but once you learn that, it's easy to manage.
You theoretically could also switch connections between receiver and Motif, but that's a pain, and using a Y cable to connect both directly to the speakers can cause weirdness because the two sources aren't isolated from each other.
The speakers should sound fine Whatever you use for a center, unless it's identical, will likely have a different frequency response and how well center and the left and right will blend for 5.1 content is partly personal preference, and partly luck. BUt I've lived reasonably happliy with quite mismatched center and left-right speakers when that's what was available to me . . . FWIW.