In my 125 reef I have a mixture of leathers, acro, mushrooms, etc... I have at least 50/50 with leathers/mushrooms slightly predominating the acros and as long as they don't touch, have not been a problem. Even when I cut the leathers (i.e., to frag or propagate) the resultant theoretical "poison" in the water has not harmed the acros (in that they have not died and they do grow). I don't know if they would grow faster if there were no leathers and shrooms, you would need a randomized trial to determine that.
I have had one
acro colony meltdown that I attribute directly to a wayward anemone majodo landing in the middle of it. No acro meltdown related to leathers or mushrooms in the tank otherwise, as far as I can tell.
My tank is over 2 years old and has always had a mixture. I got many of my acros and leathers from a guy who kept them in a mixed environment for at least a couple of years before I acquired them. I use 2, 250 W MH and 4, 140 W VHO for lighting.
My point is that I don't think it's a hard and fast rule that leathers and acro don't mix, at least in my 2 year old environment.
BTW, In my 22 gallon nano, I use only compact flourescents 110 watts total, and I grow the leathers and shrooms fine. That tank is only a year or so old. Actually, I have a frag of acro very high up in that tank that is slowly growing too

. But it's a green branchy variety that seems to be very hardy. Haven't tried any other acros in that nano because of the lighting issue. Maybe I should
My belief is that once you have the basics that are adequate, the secret to everything is good water changes. However, my stuff doesn't grow as fast or with as brilliant colors as some of the more hard core acro guys in TCMAS. So please note that I am only talking about "adequate" (i.e., slow growth, good
polyp extension and otherwise happy looking stuff). I think to get the fast growth rates and optimum colors that guys like Bill, David, etc... get, you need to be a bit fussier. Not that you guys are "fussy", but you know what I mean!
Robin