As these are still growing I would feed some concentrate, and Allen & Page All round goat mix is probably as good as any, if a bit pricey... but if you only have two it also doesn't make much difference. The best way to prevent urinary calculii is to make sure the boys drink - I give all my goats warm water and replenish with fresh warm water at least twice a day. If you live in a hard water area putting a small slurp of cider vinegar into their water also helps, though I have found that my boys don't really like it. You can also add ammonium chloride (just a sprinkle I understand) to their feed, though I have never done this. Feed for rams/tups often has this added already, and may be a cheaper option to feed to them anyway. If they have a red rockie they should be ok for Copper as well.
I have no idea how much 17 oz is... mine get a handful three times a day. But also browsings, fresh grass, some Readigrass in winter. Wethers kept as pets don't need to have continued feeds of concentrate once they have stopped growing, as they will tend to grow fat.
It probably all sounds a bit confusing at the moment, but you will learn as you go along.