You didn't buy a really nice AR so you could go cheap on ammo. I shoot steel case in my mini 14 sometimes, it shoots fine, but I prefer brass. Some AR 15's don't like steel ammo. You can try it, it won't hurt anything, but you save like 4 cents a round and I think you should just stick to brass. A box of brass .223 is like $7.99 and a box of steel is about $5.50. You don't save a whole lot.
I met a guy at the range who had a dream ar 15 with an eotech sight. It was easily a $1500+ build. He bought 1000 rounds of wolf ammo and his gun didn't like it, so he offered to sell it to me for $4 a box. I took $20 worth and had fun with it. You will just have to try it and see if your gun shoots it.
American eagle is pretty good, I shoot a lot of that through my mini 14.