The 15th Century is one of those wargame periods where sieges, large and small, and the small war of raids and ambushes form the bulk of warfare with large pitched battles being much rarer. It is entirely possible to resolve each of these types of action in a different way, different rules, different figures scales, maybe a boardgame siege and so on but for me there is something lost in that process unless taking each historical event as a stand alone project unto itself. For an ongoing campaign or pseudo campaign, it feels more natural to use the same basic rules and the same figures despite the inevitable practical concessions, skirmishes with too many figures, battles with too few figures fought on too condensed a battle field and so on. Despite all the theoretical issues, from a player feeling point of view as opposed to logical analysis, it feels right. It feels more like you are there and after all, in life the way you load and fire a musket and so on does not change with the size of a battle, nor do human emotions.
My best experience of the "rules for all seasons" approach has been my participation in the Not Quite Seven Years War. The games have ranged from small convoy actions and advance guards with a few companies on either side, to pitched battles with over a thousand toy soldiers a side, to the siege of a fortress, all using the same figures, organization and Lawford & Young's Charge! rules (The table top siege game had different rules for the prolonged siege aspects but sorties and assaults were resolved using Charge!).
The18thC was, however, a time with well established tactics and long term organizations, at least up to the battalion/regiment level. The battalion was a battalion regardless of the level of action. The early to mid 16thC the other hand saw most armies organized no higher than the hundred or so man company or troop, these often being of mixed pike and shot for the infantry. Some infantry were gathered into larger 1,000 to 2,000 man regiments in some armies but there is little sign of these being tactical units in most armies and what limited information there is on the pitched battles suggest that many armies fielded large blocks with sometimes as many as 4 or 5,000 pikemen but that sometimes these could also split without any hint as to how. Many of the shot fought as skirmishers but not all of them, all the time, especially under Spanish command but before the advent of the Tercio there seems to be little definite information on how it was all organized and controlled in practice. First hand accounts are tantalizing in the pictures they present but like most such throughout time, tend to assume that the reader either already understands the nuts and bolts or else doesn't need/want to know. Throwing in the Turks doesn't really help at all.
This tactical fuzziness isn't too much of an obstacle if wargaming at the highest levels as it can be similarly abstracted out and the main bodies of troops we read about represented as blocks of miniatures with various characteristics. At the lowest wargaming levels information is so vague that we can pretty much make it up and just field "units" based on our limited information backed by reasonable assumptions and practicalities. This is how we tackled the relief of Haddington game as described in Battlegames Issue x. We found estimates of numbers of various troop types and a few mention of troop strengths under certain leaders and guestimated from there in such a way as to make a good game while still keeping within the limited historical narratives that we had. The rules we devised worked well enough then and in various subsequent smaller games despite my tinkering but if we were to quadruple the number of stands, I am extremely doubtful that the result would ever be a credible refight of Pinkie if played as a game.
Since the only future 16th Century games that have been contemplated so far are the Camisade of Boulogne, Malta and Vienna, or the standard sort of little war scenarios, it may be a moot point but it all needs a bit more thought on the best way to proceed.