Now for your questions:
1. How do I not only show dominance over him, but the entire herd, so that they don't harass him when I bring him in and send him back out.
Follow through with your threats. Back off when they listen.
2. How should I separate him from the herd when I want to bring him in so that they do not follow, but HE will?
This is a fun one. Usually if my horse is playing hard to get I have a pocket full of treats (for AFTER he is caught and we are out of the paddock) and a rope halter. I chuck the rope halter (and hang onto the leadrope) to make him run off. Or use a lunging whip/dressage whip/whip with bag) until he stops running and looks at you then softly walk towards him. If he looks away or turns away chase him off again. The other horses I just growled at or prompted to move away. It's a pain but it works
3.I switched him from sweet feed to pellets shortly after I bought him because of his hot attitude. I thought that he had come along so well that I could try a regular feed again(as this is all my new barn offers)I am wondering if this is contributing to his hautiness?
You never know... it could be but I feel it is more behavioral.
4.The barn does not have a roundpen or arena where I can lunge him. Does anyone else have to deal with these sorts of circumstances and do you all have any suggestions on how to work with an uppity horse where the space is limited and seems to be crowded with other horses?
Find somewhere flat and use a leadline and do some ground work with him. "Send" him places, tach him to yield to pressure, to walk and trot in hand, etc.
"Strength is the ability to use a muscle without tension"