IMO, run the GH and your normal AAS cycle and hold off on the slin. At your age, and at 12%bf, if you go the slin route, you might be bitting off more then you can chew. If you do go the slin route then you'll put on size, but there is a chance of added bf along with it, especially in the long run.
My advice, spend a year with the GH and AAS, get your bf a little lower, say around 8%, and if you're still hell bent on slin, then reconsider it. All in all though, slin should be reserved for those who compete at the pro level or the national level with realistic possibilities of turning pro.