Great idea for Rohan scenario. In Gondor scenario though in my opion, only good if played in conjunction with my suggested house rules (see Godor for Experts). Gondor needs absolutely no help with the base rules.
@veldrin: I do not think that this lower bound is necessary. Since you need to score at least twice, it will already be hard (1 out of nine times) for an army consisting of two units (impossible, of course, for an army consisting of a single unit ).
genghissean: I must admit that I only played the Rohan scenario until now, so that I cannot say if I would apply this house rule also for the Gondor secnario.