Mason
If it turns out there is just a slight difference use the cardstock under the figures base idea.I was always a bit skeptical but I used it for the first time this morning and,when there is only a small height variance, it actually works quite well.
I used to be a bit of a size/scale nazi but trying to do VSF with so many different manufacturers has made me become much more lenient.I now happily mix Front Rank,Perry,Foundry,Artizan ,Mutineer ,Great War and what ever else I need.
Size differences in a comparison pic are very different from how they look on the tabletop,especially from 2 feet away 
Pete
Fitterpete; Thanks, mate, it should be fine as I compared the Boot Hill to the 1st Corps that I have, and the difference will not be a serious problem.
Yes, the BH are bigger being modern, but not in any problematic way. They are a little chunkier but will work just fine as I will be using them as unit leaders and character models, so will be quite fitting in a cinematic kind of way.
A note on the Boot Hill miniatures, (and sorry for the delay in response); They are great sculpts, full of character, and very clean castings.
Also arrived on my doormat within 48hrs of placing the order.
Fantastic!
Thanks, Malamute!