So what if RDM has a diff aesthetic to the other casters?

SMNs have had to deal with sharing gear that suits BLMs appearance wise, and likewise SCHs with WHM, MCH with BRD, DRK with the other tanks, etc. RDM is unlikely to be the exception here. Like the other Jobs they'll get their AF gear and beyond that will share Caster gear like SMN/BLM do, regardless of design.