When I'm coaching, I like to have a mix of skills in the game. Otherwise, the games are pretty much always a 5-0 for one team or the other. It's really hard to coach 6 completely new players without making it kinda unfun. I find it hard to allow aggressive play because of how out of hand it can get with multiple people overextending and stuff. With some UGC people thrown into the mix, you get a group of people that can do their own thing, so you can really focus on teaching the newbies how to do stuff during the fights. Then, at the end of fights you critique the UGC player's playstyle. Also, in groups with just noobies, the punishment for major mistakes just isn't there. With mixed groups, the noobies get the basic, the ugc players get more complex advice, the game is less chaotic, the UGC players get good at shutting down people playing overly-aggressive, the noobs learn the over-aggression has consequences, and I don't get an aneurysm. Everybody wins!
I also like coaching while playing for a sort of similar reason. Not only can I balance the teams by handicapping myself/ tryharding, I can also focus on what the other people are doing without having to work with whoever would otherwise be on the class I'm playing. It's much easier to coach 5 people than it is to coach 6 people.
Edit: Also the more IM/ Invite players that coach, the better. Give it a shot!