I too believe that they will all graze on all surfaces at about the same level of efficiency. However, if you have a planted tank, they may eat the plants more than the glass/acrylic. I also find that 1 phoenix will do a much better job, than 2-3. If you're keeping more than 1 in a tank, then I'd recommend 6+ to even out the aggression; as they're somewhat aggro to each other--more so than to other species of fish.
I'm currently housing 12 of these fish; 2 large and 10 tiny.
PS. A phoenix may not eat as much algae, if you feed it too much 'good stuff.' I will go through spurts of overfeeding and underfeeding my phoenix, in order to attain a balance between fattening her up and cleanliness of the tank.