So you used a wired master flash within the Apollo and optically triggered the others with it. That would sound like my kind of starter solutiion. I could even use my chinese flash from the cactus and optically trigger the Sigma with manual control over the output in order to tune the result. In extreme cases, I could even add my third small flash on the second cactus receiver to give even more power on my key light or as a rim/2nd light. Not the optimal setup, but may be workable to start with.
Outside yes, I used a long ETTL cable that I made myself and had the key light on it as the Master inside a softbox and controlled it via the camera menu. It then controlled any other flashes in A, B or C Group.
Indoors there is no need for the physical ETTL cable link, even standing behind the softbox the optical triggering works pretty well, but I always used an on camera 550EX as a controller and they have full tilt and swivel control to line the head up best for bouncing the control light, I don't know how well a pop up controller would work, but my ways both worked very well.
I also used Yongnuo RF-602's on occasion, but mostly outside in dymanic conditions, an assistant with a softbox mounted 550EX at a function, for instance, or for real estate interiors where I was really tucking remotes away in corners of wardrobes or other rooms and hallways to even exposures.
It was all these different scenarios that pushed me to invest in the newer RT system and I now just have one speedlite system for all my varied shooting situations, no cables, triggers, receivers or transceivers, nor the additional batteries! However for simple portrait shooting situations the optical triggering system is more than capable of dealing with flashes inside softboxes.