Gah.

I wouldn't want to go into this whole size/scale thing
again, but to sum it up very briefly:
- "XX mm" doesn't mean a thing, or its meaning is highly subjective.
- Same for "true", although it approximately means "smaller than something".
- Same for "heroic", although it approximately means "larger than something".
- Height scale, extremities scale, equipment scale and general beefiness work in mysterious, almost unrelated ways.
That said, post-90s GW stuff is a bad reference for anything, because it's more like ultra mega super absurdly heroic 32mm. Especially their plastics have gone through a dozen steps of scale creep. They're "28mm" minis in rubber suits, wearing platform shoes and carrying half-tonne weapons. Nothing to do with real or typical tin anatomy.
Another serious offender is Reaper Chronoscope, where the scale is chaotic and on average well over anything that could be called 28mm by actual measurements. They can be used, but most of the minis must represent noticeably tall people.
Other than that, you can quite safely mix Grenadier, RAFM, Metal Magic, early Citadel, Artizan, Pulp and pretty much anything on the market. Weapon scales will vary slightly, but the overall look should be generally acceptable. And you can call that scale anything you ever want, because "XX mm" doesn't mean a thing.
For actually useful information, I recommend scale pics. I posted some
here a few winters ago, and I can take new ones some day with many more companies - including a couple of different cultists.