I've hear ya man, I was initially turned off because I've heard everything you said but now there's a small conversation about growing on the back glass and I think I can make that work without it spreading as much as I worry it will.You're worrying about GSP far more than is necessary. Keep it isolated from the rest of the tank and aggressively trim it back if it even thinks about leaving your island. If the mat gets over onto your rockwork, say goodbye to that rock for anything other than GSP unless you do something tank-killing to it. GSP grows out of a mat, and said mat will force itself into the tiniest nooks and crannies of your rockwork. If you do manage to peel the mat off your rockwork, it'll grow pretty much wherever you put it. Honestly, you have to really try to kill GSP.
Acan lord corals (might also be labeled as micromussa) are always a good addition to that kind of tank. Lighting demands aren't too intense, and you can always feed them to supplement what your lights don't provide - and you should feed them, if only for the fascination of watching them eat. Tons of different color morphs available, though most LFS seem to get mostly reds and teals. I've seen them in pinks, purples, greens, blues, orange and more.
Also ooooh! Acans look cool. Will def look for those on Wednesday, thanks for that advice!