I would like to confirm:
optiones - no proof for special headgear. the hastile should be enough
optiones were chosen by the respective CO to fulfil special duties, in the case of the O.centuriae as second in comand - I would understand that as a kind of warrant officer comissioned by the CO
signiferi were a promotional rank of high esteem - I can't imagine a combination
as to the optio draconarius - the Late Roman army is very different - he might have been simply "chosen from among the standard bearers"
I don't see a problem in using standard bearer miniatures with special "optio helmets". The real optio would have the staff that defines his function in the same way as the vitis stands for the disciplinary power of the centurion
nice miniatures btw