I actually had a similar idea to let my zoa colonies grow. Currently, any frags/colonies I get go into my display and a few polyps are fragged off immediately and mounted on a small 1x1 tile that goes into my frag grow-out tank. Once the small tiles start to fill in a bit, it's mounted on a larger 4x4 tile. I then use these as my mother colonies and frag a few polyps off each month that are mounted on small frag plugs and grown out a few weeks for resale. By simply added a tile with a larger footprint below your existing tile(s), you give you colonies a nice surface to grow out on and it makes fragging them MUCH easier since it's a uniform surface. This also lets your mother colonies grow in both height and width.