Hi David,
I think that the rules can be easily adapted to other scales. The only thing I´d make sure is that all figures are based following the same convention. Now, I wouldn´t use 25mm bases for 20mm/1:72 figures; 20 or 15mm are better, and should work well; of course, you´ll have to take care to space out your troops if facing artillery. I´m using 15mm for all my 20mm stuff, and to date, it worked very well.
I recently did some concepts on adapting our rules for 20mm Vietnam (which is my latest fad), and calculated some aspects. Generally, movement rates and weapon ranges do not need to be adjusted too much, but I´d suggest possibly reducing the close combat ranges (which were deliberately scaled with 28mm figures on 25mm bases in mind). But that´s a minor thing.
15mm, now, that might be a bit different. Again, less of a problem if you have single-based figures, but if you have "stands", you might have to do some converting, because T&T isn´t really a mass battle system. Otherwise, just treat each stand as a single figure, and have the "large scale" on an aesthetic level.