Hello,yes, that may be a idea to add a card or a plate and paint the missing figures on that.
For myself I perfer 30mm figures. I have painted one or two bigger ones but was not quiet happy with that. It is interesting to see how this all develops, how more painters switch to bigger figures and paint them like real art works on a very high level. Thinking of my own painting I´m would more think of craft then of an art.
Christoph