As wargamers we all like our clear delineations between period and equipment, but are also (generally) aware that things weren't always that clear-cut. I like to have some variety in appearance on the tabletop, but nevertheless I do have some nagging concerns about equipment and whether or not it's appropriate for the period in question.
Frustrating in this respect is the tendency of some authors to write that, for instance, the Gallic helmet first appeared in the 1st century BC. As someone who is currently building an exclusively first-century BC Republican army, it matters very much whether that helmet design appeared towards the start of the 1st century (in which case I can use it with impunity) or only at the very end under Augustus (in which case I shouldn't really be using them at all).
I know that often we are extrapolating from a very small set of sources. The number of times I've been shown pictures or read descriptions of the Altar of Ahenobarbus alone… And I know also that we are often arguing from absence, which contributes to the vagueness. But still, tabletop convention aside, it would be nice to nail down some guidelines for what I can get away with.
So my three principal concerns, as indicated in the title:
When did (rank-and-file) legionaries stop wearing greaves?
When did the Gallic helmet first see use?
When did legionaries start using squared-off scuta?
The greaves are mostly relevant because they increase my options for acquiring good figures relatively inexpensively (as the Victrix republican legions can easily fill in for Caesarian-era legionaries otherwise).
The shield I know is pretty much a toss-up. Goldsworthy indicates that while it's generally been assumed it only appeared in the first century AD there's no real reason to do so, given the gap in the record. Nevertheless, if anyone has any top-secret information on the subject, I'd be glad to hear it.
I've got about as far as I can with my own research. JSTOR has failed me, though that's probably my own fault for putting in the wrong search terms or something.
I realise it would be pushing it to have a legionary with a Gallic helmet, a rectangular shield *and* greaves. I wouldn't do that, unless I were modelling some imitation legionaries who hadn't done their research or something. But it would be useful to know if greaves were (potentially) still in use during, say, the Marius and Sulla period, and/or whether the Gallic helmet and rectangular scutum had appeared by the time of Pharsalus.