Don't know about Tangent, but the CP models tend to fluctuate in scale from 28 to 32mm. You should be able to get an idea from the photos becasue many of their examples are on 30mm round lipped bases. The real question is why worry? They are aliens and would surely come in many different sizes.
True, although even a multi-species military unit is likely to have as much uniformity of equipment as varying physiologies will permit. The troops may vary a lot, but you'd expect armor and weaponry to have shared design features and be of broadly similar sizes, with any soldier capable of using your Mark 1 basic zap gun being issued a Mark 1 basic zap gun even if it isn't a perfect fit for their body shape/size.
Just thinking about the logistics of supporting a really diverse set of alien species in the field is enough to seriously strain suspension of disbelief, and mixed units on the squad level would be a quartermaster's nightmare.