I actually raise and show Nigerians ( a dairy breed) and have been for a while so I have a lot o experience with goats :)
I would feed grain. But just a little ( like 1 or 2 handfuls). Since they are 5 months old they are still really growing. I would feed them a GOAT grain ( you can get it at TSC). Purina Noble goat ( TSC) has ammonium chloride in it which prevents Urinary stones and such. In the picture they appear slightly underweight to me. With big bellies- they may have worms. Ivermectin no longer works with goats usually so I would use either Cydectin or Panacur,though I don't know your area. Just make sure they don't get fat. ( not good for wethers.)
You can stop feeding grain in a few months and then just feed it in the winter. The amount of hay you are feeding them sounds good. Though I feed all my young kids free choice hay. You can feed them fruit and veggies (check to see if they are poisonious to goats)
Goats actually do much better with a loose mineral and need a mineral especially for goats as they need a high amount of Copper.
Make sure you trim hooves regularly.
Well, that is all I can think of at the moment but you can ask me any questions.
Chocolate is the answer. Who cares what the question is.