The TB Line and Pendraken EMM and ELM as well as AB figures work very well with each other - which offers up just over 100 stock codes (not counting flagsheets) for 10mm Medieval. Magister Millitum are only slightly less in heft and size of the Pendraken range (imo), so every MM figure I have seen mixed in nicely, should you want something from them. Kallistra are really too big to consider and will definitely look out of place.
To address another comment: I actually don't consider Pendraken to be "large", they are a touch heftier than other productions, but that kinda helps to make them more durable, and they are otherwise pretty true to the 1/150 stated scale, IMO.
"The TB Line figures are probably the nicest ones available in 10/12mm - but all the detail can take some effort to paint." <-- I completely concur with this comment, though I don't find the detail an issue personally, I can see where some might wince at separate shields.
As for mixing Old Glory and Pendraken, their individual figures may be ok for you (not so much for me), however the strip figures definitely won't mix well IMO.
Just my .02 and I see Pendraken figs every day.
Disclosure: I am the distributor for Pendraken in the US.