I tend to game in 10mm, but my thinking should apply down to 6mm well enough.
I’d suggest going with 30mm square bases for the British, with a bit of extra depth as needed for wagons - I don’t think you will need anything like 80mm depth, a 10mm wagon can fit on 20x40mm base, and nearly always will fit on 20x60mm base - unless you are planning on multi-basing your wagons.
For Zulus round bases might work better to give you more irregular formations - I think I went with 25mm bases for mine (aka 2p coins).
But what really matters is the look of the units to you, so I’d suggest getting some figures and blu-tack, and cutting some cardboard squares and rectangles, arranging the figures on them, and seeing what looks good to you.
If you aren’t set on rules, then square bases are probably a bit more flexible across rule sets, but basing seems far less critical in colonial period rules than in ancients / medieval ones.