Right now, unless they are milking and are therefore on a proper milker's ration, only very small quantities of concentrates are required. Feel their spine (as would with sheep) and there are guidance drawings on condition scoring goats on the web too. You want to be able to feel the transverses, but not too sharply - get a copy of David Mackenzie's Goat Husbandry (5th edition) via
www.abebooks.co.uk. It is easy to get GG's going fat - much easier than BT's for example.... Also a good handful of horse/pony carrots (chopped into thin sticks rather than slices), or cabbages or bananas, apple slices (not too many - quite acidic) are welcome. Mine do not get concentrate at this time of year, just some soaked sugar beet shreds with a dusting of rolled oats. Concentrate will be re-introduced 6 - 8 weeks prior to kidding/milking. Growing kids will still get a good handful of baby calf mix or dairy nuts until the end of their second summer ((just before mating) as they are still growing. Then diet!
Once they are in milk you may want to try feeding them on dairy cattle/calf feed (either mix or nuts) if you can get it up your way, if not sheep mix is fine. You will need a Red Rockie lick in any case, they will ignore it for most of the year, but mine do lick it when they are in kid and when milking first. Goats do need higher copper than sheep do, and the RED Rockies (not the yellow ones!) will provide that. Sheep should NOT have access to the red rockies though, same goes for cattle feed. If you have sheep and goats together it is safer to stick to sheep mix. But as there are cattle on Shetland you should also be able to buy cattle/calf feed.
Hope this helps...