Roger,
Legality wise, I've assumed they were legal, I don't see why they wouldn't be but as ever PW is the man to ask.
Motor range wise I'd say,
4.5(any) unlikely to be used, only on a crap club/regional track that is far too big. At Southport, even with it's massive straight I had no plans to motor up from the 5.5R in 4wd because as the Novaks are sintered I just geared up a tooth, they tolerate a fairly wide range of gearing.
5.5R is unlikely to see the light of day in my 2wd now the L's are out because the R's have so much grunt that it's only ever going to make it more hard work getting on the throttle than the equivilent L motor. Having said that, the 5.5R is the bread and butter of 4wd, it just never get's changed. You're right that on wet astro perhaps a 6.5 might be better but even last year I don't think I ever did that (though it may have been wise).
5.5L likely to be my 2wd motor of choice for big tracks (Oswestry national for example, maybe Southport), controlable at the bottom end but winds up freaking fast in a 2wd.
6.5L is the motor I would run most of the time on any track up to and including a 'medium' in 2wd. Again the L over the R in 2wd just because it's smoother/better getting on the throttle. This isn't quite the fit and forget (that the 5.5R is in 4wd) but I imagine it will get run most of the time.
7.5 (any) I've only ran a couple of times, really nice and smooth and super efficient but if I'm honest I'm always tempted to motor up to the 6.5L. As you say slippy, cold wet astro (current outoor conditions) then this may be the best but I race indoors in the winter (girl - I know) I don't really know.
So I'd defo get a 5.5R to run almost exclusivley in 4wd and a 6.5L and 5.5L to run in the 2wd. If you're happy to get a 4th and do run in SUPER slippy conditions (or tight indoors) then a 7.5 could be good, but I've not tried the L version at all or used the R version that much so can't recomend here. I think you'd get more value out of a 5.5L for the 2wd than a 4.5 for the 4wd, also save on rear tyre bills
Actually I remember reading someone else saying they thought the L motors have the bottom end of the next motor up(numerically) but the middle and top end of the same numbered R. So a 5.5L has the bottom end of a 6.5 but the middle and top of a 5.5.
Dan