I would not feed - or feed very little alfalfa or other high energy feeds to an easy keeper. I'd stick to something like hay/grass & a low-carb nutritional supplement. For your young arab, remember he's an arab, not a QH for one & they are finer built. Also re growth spurts, I wouldn't stress at all about him 'losing weight' then. If you look at other animals - puppies & human kids for eg, they tend to go through stages of looking stocky & then looking lanky when they have an 'up' spurt. It's also healthier for them not to be kept overweight, as with us
Therefore I'd probably be inclined to feed them pretty much the same & see how they go.
I would generally avoid grainy, high starch feeds, especially corn, which is particularly high in starch & harder for horses to digest than other feeds. Both Omolene and Calf Manna(that has 2 types) have corn in them. If you're going to feed something 'high octane' such as these, it's even more important to feed little & often, to minimise risks of this type of feed. Otherwise I'd go for something like beet pulp, which you can mix supplements with. As for oil, I tend to feed ground linseed, which provides some oil along with essential fatties. This does lose it's nutritional value quickly after processing though, so I buy the whole seeds & grind some daily myself.