Searching For Scabbard and Gladius PARTS

[Crispvs]

For the optio impression you needn't look any different to a regular legionary really. The only things we are aware of which distinguished an optio visually from the other soldiers were his optio's ring and the knobbed staff he carried. Even the well known side plumes worn by many re-enactor portraying optios area modern assumption. Many helmets exhibit fittings to mount feathers but this may have more to do with unit identification than rank. This might not seem like much to distinguish him visually from his men, but in the same way men in a modern military unit know who their own NCOs and officers are without necessarily needing to see their rank badges, Roman soldiers would have known perfectly well what their own optios looked like without them having to wear special crests or armour to advertise their presence.

In answer to your question, yes concentric circle plates would be fine for the AD25-50 period.

A problem you are likely to find with your proposed officer impression is that we have little solid information to go on for the appearance of officers above the centurionate, apart from the fairly conventionalised appearance of imperial statuary in military guise, which may or may not resemble actual equipment.
There is also the issue of whether you want to portray your officer in the field or in everyday life. I think it likely that most officers (and here we are talking military tribunes, the general/legate and possibly the camp prefect) would have worn togas when in ceremonial roles and possibly even on day to day duties.
What they wore in the field is anyone's guess really. While the muscle cuirass and mail faced with scales are both possibilities for body armour, along with the possibility of greaves, we really have very little idea of what sort of helmet an officer would have worn, or what his sword or belt might have looked like. There seems little chance in my mind that a general would have taken a parazonium onto the field of battle - such a thing was probably reserved for the same sorts of ceremonial occasions when regular soldiers and centurions would have paraded with the honorific spears, crowns and other non-battlefield paraphenalia which could be received as rewards for bravery or good service.
The only things we can really be reasonably sure we know about for officers are a narrow cord or ribbon which was tied around the body with a particular style of knot as a symbol of command rank and the particular styles of footwear appropriate to the equestrian (the narrow stripe tribunes) and senatorial (the legate and the broad stripe tribune) classes of society.

Sorry to muddy the waters for you.

Crispvs