As long as the troops for both sides are the same size, it does not matter.
This is easiest with Skirmish and DBA type games, as fewer figures needed.
I have a matched pair (Carthaginian & Republican Roman) for DBA. both sets
of figures were sold as 25mm. I am currently garnering Hinchcliffe (25mm)
Ancients for more 'Matched Pairs' by picking up (cheaply) figures from e-bay.
This is a bit slower than ordering a complete 28mm pair of armies, but I have
plenty of other projects to keep me busy.
