Yes, absolutely, mix and match.....I used Essex figures in Evil Gong wagons. Plus another brand (forgot which) for artillery etc. It's actually an army suited to picking and choosing from different ranges.
My recent order of Museum Miniatures included extra wagons (WW03a) to build a Hussite army for the final crusade of 1464-1471. I have four wagons in use now and these have Legio Heroica figures as crew.
The recent additions will have a mix of Museum medieval shot, crossbow and halberd of other nationalities for the crew and four blade.
I’ve used Medus Wwg for my Medieval Germans and they are great, even if they need a lot of super glue...but they are very flexible and allow for lots of options to be mixed and matched. However, when it came to Hussites I went for the greater historical accuracy and bought Museum Minis,mostly to have a common look and feel. That said they have lain unpainted for a couple of years while the ones from Alternative are complete.