I'd do what you're doing. I have no idea if it will work, but it's seems to be to be the best way of going about it.
Everything I've read about large, more aggressive cichlid species is it gets more tricky to add new fish once they're full grown.
My recent experience is limited to Severums, Chocolates, and assorted Geophagus. In those cases it was never tough introducing new fish... the only thing I haven't tried is another chocolate, and of all the species I keep currently, the chocolate is supposed to be the most aggressive to conspecifics.
Anyhow, I'm in the process of listing my current stock for sale locally so that I can try to get a nice harem of GT's!
It's pretty cool that you have wild caught GT's. I've read they are from Ecuador and it is supposedly extremely difficult to export wildlife from there. That would explain the $$$, or in your case the £££. (I am Canadian and have no idea what a quid is!!! I'll Google it!)