I think intermixing systems is (one of?) the dirty little secrets of this profession. Generally speaking, manufacturers instruct against it, but lots of people do it (and not just with gel/polish). In the DIY universe, we do it all the time, but I would look askance at a pro who did it.
Gelish advertises that its base and top coats can be used with any system, but CND Shellac says no to that. Yet Shellac, with its easy removal and fewer colours, is one of the most sought after brands for intermixing.
I think that this is one of those issues, along with the lamp issue, that will have to be resolved over the next year or two. When there were only a couple of manufacturers, "stay within the system" and "use only our lamp" made sense, and manufacturers could dictate the terms. But now, with dozens of systems out there and techs using multiple systems, it is cost prohibitive to have the lamp for every system. Two years ago, there wasn't a lot of choice in terms of systems; now, if one system doesn't do what we need, we can just move on to the next. Similarly, clients more and more want us to mix systems to get the best of each one - the easy removal of CND Shellac or OPI, the acetone resistance of Gelish, the color choices of ACG or Just Gel, the matching polishes of LeChat, Orly or Geleration. It is properly the role of professionals to customize their services to the benefit of the client. But how much is too much? CND has made it clear that for Shellac, using another lamp or intermixing is too much. Gelish says go for it. What is your liability in intermixing? I think these are the questions that techs and manufacturers will have to resolve in concert.