And of course, there is no reason why any rule set that works with 28mm, or 6mm, or anything else, won't work for 15mm - you might want to shorten the ranges, but it's usually not necessary.
I use Sharp Practice 2 for my ACW games, it's aimed at large skirmishes and is a marvellous set of rules (tbh, it's my favourite set of rules by far). I've reduced the distances by one third, but only because my gaming space is smallish; there are quite a few people playing SP2 in 15mm using the distances as written in the rule set. Actually, with 15mm, ground scale matches figure scale.