The 20mm are smaller, but stick a bit of card to the base and they sit well enough (tried and tested with the Colonials).
Of my several thousand painted figures, I guess about 40%+ are metal, from several different manufacturers. I think, unlike some of our members sticking a new toe in the 'mix 'n' match waters, I can claim some expertise.
I agree with your post, Alan, and would suggest it applies also to Naps, Ancients and WW2 as far as Newline is concerned but also as a general rule of thumb for most small disparities in size between ranges.
My advice is that girth matters more than height when matching figures. You can, if necessary, conceal differences in height but if some figures are notably heftier or slimmer, this is far more obvious & hard to hide.
I should also add a lot lies in individual taste: what I can live with or reject may well differ from your tastes.