I've run a lot of 4 player (2v2) games of R&P, and by necessity have therefore playtested a lot of R&P playing two companies on one of the sides (sometimes both sides!). I usually have the perspective each unit is a company anyway, like Vodkafan, at its base level.
I've also bathtubbed it up and done Aliwal as a 4 player game, with each unit as a battalion/campoo, and I've sort of done the same with Pikeman's Lament. When I've done that, I've halved movement and firing ranges, to give a sense the table was bigger. It works ok for a fun night's gaming, but our Bloody Big Battles version of Aliwal felt better. The difference was the players for R&P were casuals, it was a harder core of us for BBB.
When doing a 4 player game, I usually keep it at 24 points per force. I once contemplated a 6 player game, and I would have reduced it to 18 points per player for that.
Chris