It's a question that constantly comes up - "Will _these figs_ look ok in the same unit with _these other figs_?" And it's a good question. There're significant issues with relative figure sizes that are obvious - some figures are just plain taller than others. UK seems to love 28mm figures, while US still has a lot of 25mm figures, e.g. Old Glory.
My opinion is that more important than height is the sculpting style. Bulk especially is more obvious than height as it catches the eye. Consider these 40mm scupts from Romanoff and Sash & Saber:
Of more visual impact than the height is the bulk of the figure. This is partly a sculptor's choice [therefore fall into "art" category for me] but also the Romanoff has longer legs / higher waist. Certainly people come in different shapes and sizes, but across a line the figures tend to be very consistent, usually from the same sculptor.
Consider the mix into a unit. This unit is from some time ago when I was pressed for both figures and time. I was throwing together some War of the Roses armies for Terry Gore's Medieval warfare, and I needed an 24-figure unit of 6 bases, 4 longbow and 2 men-at-arms. From a distance, they look fine. But closeup...
You can see that most of the men-at-arms are smaller, as well as having much smaller hands and faces than the archers behind them [not to mention the fact that they're from different periods - archers are 100YW and MaA are WoR]. The large hands and faces is to give the figures more features at a distance. Up close, they look like Charlie Brown with oven mitts!
The difference is even more obvious in the side view:
The bulk and size of the bowmen makes them look like at the least Heroic 28mm scale! They are from some unknown company to me, I think they're actually "fantasy figures" who were in a pack of 10 with the label "City Guard" or something. They remind me of Grenadier, but as there are two Grenadier figures supplying the comparison MaA I know it's not them! Notice that not even the greater bulk of the Grenadier MaA than Foundry or Old Glory helps them make up for the height distance, nor the large faces and hands. Who are these giants anyway? I've no idea, but here's another pic:
Anyone with info, shoot me a comment!
Anyway, sometimes, size [or bulk] really does matter. This unit just doesn't look right to me. Can I make the 40mm stuff work out together, I don't know. Chris' Sash & Saber style doesn't gel too well with the Romanoff in this shot.
Overall, I think that a unit based together needs some sort of internal integrity, more than just the paint scheme. As you can see above, a color scheme doesn't make the figures match up well.
Amazing what I find in my old boxes! Some of this battered old stuff should really get put on the market, I think.