Yes great idea, can you train them to only eat the Coralline off the glass and not the rocks?Why not just try and get a few sea urchins and some limpets. Both of those should eat up that coraline no problem. If that is the only downside of course.
Also a good idea, montis and cyphastreas are great for this!
It's starting on mine the little circles of coralline are growing cover pretty fast. I have Monti cap frags growing on the bottom to shade most of it until I make up my mind on cyphastrea v. Encrusting montis.
cyphastrea do great in low par. I grow mine at the bottom of the tank partially under a ledge. 24 inch deep tank with orphek atlantek V4 set on soft coral/lps setting (in other words, dialed way down).I only get about 70 PAR at the bottom so Monti's are out (running 4x XR15 G4 PROS on AB+). I suspect the low PAR and blue light is a leading contributor to my Coralline growth. In other parts of the tank, I have the worlds cheapest cheato reactor going - it just sits in a big fish net under a radion next to a gyre (a temp fix when I moved house then never found the motiviation to change because it works so well) and under the net where the light is shaded the Coralline is insane, but just next to it not shaded far less so.