I've decided to (hopefully) simplify life some. I've had a 580EX II for some time which I occasionally use on camera, but most often use as a slave to a Qflash Trio, if at all. The Quantum units communicate wirelessly (RF), produce really nice light, recharge immediately, and can fire repeatedly without overheating. But they're a bit more than what I need at this point. So they're going. I've ordered two 600EX-RT's.
I hate to use a big flash as just a master/fill but choices are limited. The ST-E3-RT seems poorly designed with the info and controls at the top, not where my eyes would be, and without focus assist. A 90EX can be a master, but has no RF, it's fairly weak, and has some other undesirable shortcomings. On my wish list is something like my old 220EX as an RF master. It has more than enough power for on camera fill, proper focus assist, and reasonable recycle time.
So my question really is, can I use a mixture of optical and RF slaves with a 600EX-RT master, specifically my existing 580EX II and a 600EX-RT as slaves to a 600EX-RT? Or must I choose optical or RF?
I think ultimately I will wait for the ST-E3-RT Mark II (or ST-E4, whatever they'll call it) which hopefully will address some of the shortcomings of the current model. I may or may not sell the 580EX II depending on how it performs with the other two, and get a third 600EX-RT, but not right away.