I probably have more rules around my modular equipment now than I did at one time. But, it's more to do with the systems as a whole and how they fit into the larger scheme of things.
So, with the exception of utilities such as VCAs/Mixers etc, I tend not to have too much duplication between systems, especially around oscillators. My Bugbrand (analogue oscs) is very different to my Modcan (digital fm oscs) which is in turn very different to my euro (no oscs!).
Within each system though I tend to like things to be pretty self contained. Each needs to work in its own right.
I also have a couple of small sidecar cases that I can use with any of the systems. One with some instrument interface type modules to bring external sound sources in. One with a couple of cv to midi modules. Doepfer minicases are great.
My main other rule these days is that none of these systems will get any bigger. As some of you know I am trying to downsize!
Rules and considerations might be a bit different when you get into something like Buchla. Although given the prevalence of clones now, there are more modules available than in the "just 200e" days which is what I have experience of. Then it was a case of picking a system from a relatively small range of modules. Not to say that it wasn't a headache though - the same considerations around system balance, size and cost come into play whatever format I think. The good thing with Buchla for me was that as a format it really stopped those impulse buys that are all too easy in Eurorack. Real estate (and modules) although very fairly priced for what they are, were not cheap and much thought/saving had to go into the next piece of the puzzle.