Sorry to muddy the waters here, but we actually know very little about what soldiers of differing ranks / roles really looked like at Caesar's time.
As for optiones, as their only known symbols of rank were the large optio's ring and knobbed staff, I doubt that they would be identifiable in contemporary sculpture, as I very much doubt that the optio would have taken his staff into battle with him (I think that it might have got in the way of him being able to draw his sword).
As far as I know, the only representational sources are the Alter of Domitius Ahenobarbus, the Column of Aemilius Paulus at Delphi, The Arch of Orange, the Monument of the Julii and some figures which might possibly be Roman soldiers on reliefs from Osuna in Spain. These should constitute a plentiful supply of visual information but unfortunately I doubt that any of them will be particularly helpful in answering your question.
The Column of Aemilius Paulus dates to the 130s BC and is contemporary with the army described by Polybius, eighty years before Caesar's campaigns in Gaul. You can see it here:
http://www.romanarmy.com/cms/component/ ... temid,135/
As you can see, the figures are quite damaged, but Roman infantry are shown in mail, with belts and carrying long scuta. Cavalry are shown with large round shields. Given that at least one cavalryman is wearing a Thracian helmet, it is probable that some of the figures are intended to be Macedonian. There is not much indication of rank amongst the soldiers shown.
The Altar of Domitius Aheobarbus is of uncertain date and is variously dated between c100BC and c40BC (IIRC). It is the main source used as a visual reference for soldiers in Caesar's time, although it is by no means certain that it is contemporary with Caesar. A lot could have changed within this sixty year time frame. You can see it here:
http://www.romanarmy.com/cms/component/ ... temid,135/
Six soldiers are shown. One of these is a senior officer of some sort, so is not relevant to your question and one is apparently a cavalryman, wearing mail and some sort of Boeotian helmet. The remaining four are apparently infantry and wear mail, belts, helmets and carry scuta. As you can see, each man is depicted as slightly different to the others in terms of his equipment but there is nothing to tell us whether these differences indicate any differences in rank.
The arch of Orange celebrates Caesar's victories in Gaul and so should give us a good indication of what Caesar' troops looked like, but as it may have been built fifty years or so after Caesar's actual again we may be looking at something rather different to what Caesar's actual soldiers looked like. Soldiers have curved scuta and wear a variety of helmets, some of which seem to have horsehair crests. As far as I know no standard bearers are shown and although at least one figure has been identified as a centurio I doubt that any optio would be identifiable as something as small as the optio's ring would be too small to see, even if it was to have been depicted. You can see it here:
http://www.romanarmy.com/cms/component/ ... temid,135/
The figures on the Osuna reliefs are a bit worn and the figures identified as Romans show nothing to suggest that they are anything other than common soldiers. Another figure, playing what appears to be a cornu, may also be Roman. He wears greaves and a belt but wears no other armour of any sort. If he IS intended to be Roman, as a musician he may be the closest thing we see to what a standard bearer might be like, but the lack of body armour, shield and weapons should lead us to tread carefully in identifying his as a Roman. They can be seen here:
http://www.livius.org/a/spain/hispalis/ ... _mus39.JPG
and here:
http://commons.wikimedia.org/wiki/File: ... .N._Madrid )_01.jpg
The Monument of the Julii at St Remy was built c30-20BC so should be our best datable contemporary source. It shows soldiers with curving oval scuta and a variety of helmets, some of which are crested. As far as I can see there is no indication of rank visible. It can be seen here:
http://www.livius.org/gi-gr/glanum/glanum.html
To add to this we can also cite the stele of the centurio Minucius Lorianus from Padova. Minucius is unarmoured but carries a pugio, hung horizontally from the front of his belt. Of course, Minucius is a centurio and so that still leads us no closer to an optio or signifer. He can be seen here:
http://s129.photobucket.com/albums/p239 ... fig022.png
To add to the sculptural evidence cited above, archaeological finds suggest that Montifortino helmets would be most likely for Caesar's soldiers, possibly along with the odd Attic type helmet and probably captured Gallic Coolus, Agan and Port type helmets as well. Swords would probably be somewhat longer than those of the first century AD and it seems likely that most belts were not decorated with belt plates as seen in later periods. Some soldiers would probably carry pugiones as well. Footwear is open to question as none of the sculptural evidence tells us anything useful about footwear, unless you want to believe that they went about barefoot. Some useful pictures can be found here:
http://s129.photobucket.com/albums/p239 ... fig025.png
here:
http://s129.photobucket.com/albums/p239 ... fig023.png
here:
http://s129.photobucket.com/albums/p239 ... fig026.png
here:
http://s129.photobucket.com/albums/p239 ... fig030.png
here:
http://s129.photobucket.com/albums/p239 ... fig032.png
Well, I think that that just about sums up what we know of the appearance of the Roman soldier of the first century BC. As far as I know there is no evidence of parmae being carried by signifers before early second century AD. Unfortunately, as you will have realised by now, there just is not enough information to know how optiones and signiferi of the first century BC would have looked like in comparison of what we know of the appearance of soldiers of a century later. I think it is probably a safe enough bet to think that standard bearers would have worn animal skins of some sort over their armour but I think that is about all we can say unfortunately.
Crispvs