Hello Donald,
Black Powder is very forgiving with respect to basing and unit sizing. I myself use figures on round bases on movement trays as then they can be suitable for various skirmish games. So my Samurai, fighting Koreans, (using Pike and Shotte, actually) can be used in my Ronin games.
I also have the same irregular movement trays as you use for your Zulus, as I use for mine.
For the sake of your opponent, I would suggest that large units do look larger than regular units. That would be especially helpful if your army has both large, and regular units in it. Having said that, I don’t see any problem stating that all the Native units are large and the British units are all regular. As long as that is made clear to your opponent, things should be fine.
Have fun!
Ralph
