Hi There...the simple answer is yes you can mix. If you look at period images it is harder to find everyone dressed the same than with slight variations, once they are in the field. Although I designed new Iron Brigade figures for 1st gear/W.Britain, these will also mix well with the earlier releases with felt hats and kepis or fatigue caps. In the next go around we will add some Federals in sack coats with felt hats and/or Hardee hats too. By Gettysburg you would find a fair number of Iron brigade members wearing this last combination along side of men with 9 button frock coats. The 9 button was fairly common and these earlier releases could represent any number of units, although at the time of release I was considering Ohio troops. I suggest you pick up one of the photographic history of the Civil War books at one of the major bookstores. These period images will be a treasure trove of details for you.
By the way these will go perfectly with the Iron Brigade set I sculpted for Don Troiani...these show up on E-Bay from time to time. These in turn, will work with the earlier ones I did when I was working for Conte including the special set that came with a section of turnpike fence, and before that, the 3 figure set for Ertle/W.Britain. You should be able to combine all of these to build a realistic company. Look for additional flags and support figures later this year from 1st Gear/W.Britain.
I hope this helps, Ken Osen