My Prussians are early war WW1 Germans, my Brits are a mix of Redoubt (manning the artillery) and BTD for the rank and file. My Russians are largely Copplestone BoB and my fledgling French are a mix of Renegade WW1 French and Brigade games figures.
I'm pretty relaxed about mixing early war Germans with Colonial/Zulu Wars Brits on the same table - if technology has taken a side-street to that of our history why wouldn't military fashion?
The BTD sculpts are a little larger size-wise, but I don't tend to mix figures within a unit anyway, as mentioned the helmets are "different" but I don't mind them - YMMV.