The Gringo40s are a little smaller overall, a bit shorter, and a bit slimmer. They are truly the same scale, but are close enough for me to use them together. You can get a look at a mix of both types, plus some older Foundry figures as well, over on My Camerone Day blog, located at:
link
There are 17 blog pages worth of pics -- with an average of at least 25 pics per page, so more than 400 altogether -- from a big game some friends and I had a blast playing on the 150th Anniversary of Camerone, this past April 30th.
Meanwhile, here's one pic with a mix of Gringo40s, older Foundry and later Foundry FFL together, from the very start of the battle, when the Legion formed square to repel approaching Mexican cavalry
EDIT:
Here's a possibly more useful size comparison pic, with Gringo40 on left and later Foundry on right