They are pretty much the same size as Empress, maybe a micron or so shorter but similar proportions. About the same height as TAG, I've got a TAG Vietnam figure and a Eureka Contractor in front of me at the moment. Not sure which Portuguese you have but the ones I have are a very good match for height to both TAG ( actually they are a bit taller than the TAG examples I have).
The big difference is that TAG are much less well proportioned, they're chunkier and to be honest the sculpting is crude by comparison. I don't mind some of the TAG figures, the newer ones are better but they suffer by comparsion to Eureka or Empress in terms of overall sculpting and casting. TAG weapons tend to be fairly piss poor.
For what it's worth Eureka are a very good match height wise for Mongrel and MoFo although the MoFo stuff is heftier on the whole. The Mongrel Brits I have are a little slighter so in heft terms they are a slightly better match.
I have no problems mixing any or all of the above.