For many years now I've successfully restrained my tendency (shared with most other wargamers) to go whizzing off into new scales and periods by operating under three self-imposed rules:
Rule 1 - 20mm is skirmish scale and skirmishing is done in 20mm.
This means that I can't do 28mm skirmishing and I can't do massed battles in 20mm. This, in turn, means I can get away without having to build Rapid Fire sized units which tend to require a lot of figures and/or vehicles.
Rule 2 - 6mm is massed battle scale.
If I come across a set of big battle rules I want to play I have to do them in 6mm scale. This has great advantages in both cost and time and 6mm massed units can look great.
Rule 3 - Rules 1 and 2 don't apply if the army in question is for Hordes of the Things.
This is because (a) HOTT armies are relatively small and (b) I know they'll see plenty of use thus justifying the deviation from rules 1 and 2.
Now, however, I'm weakening. I've just had a little go at Basic Impetus by sticking a few of my old 28mm Wars of the Roses figures onto sabot bases. I've just about got enough figures to allow me to do a couple of Basic Impetus armies. In this case, I suppose I could relax the rules because I wouldn't eed to buy and paint any new models.
What's got me intrigued by Impetus is, as so often, the basing standard. At 12cm frontage, an Impetus unit base is well suited to building the army in the form of a series of dioramas. And what dioramas! Check out this site. I'm not going to achieve anything like this standard but you've got to admit it's damned inspiring!
Impetus seems, at first try to be quite fun and fast moving. I've just thrown four unit each of Lancastrians and Yorkists at each other. The charge of a unit of Lancastrian mounted men at arms into a pair of Yorkist heavy infantry units was instructive and had more flavour than you'd get from DBA or its progeny. The bill unit hit square on by the cavalry was routed immediately but the supporting dismounted men at arms fought a back-and-forth melee over about three turns before loss of the other Yorkist units gave victory to the Lancastrians.
I think I'll give Basic Impetus a go at one of our Saturday sessions but i've got a feeling there's a big rebasing sessions to come. Sod the Rules - leges utiles hominibus sunt!