Outfit is all up for grabs so don't worry too much about that. When it comes to things that you wear, you can choose what you want. Regularily, Rath'Kar wear cloth or something very light but I have no problem with him wearing actual plates. You don't have to have every characteristic of an entire race of people. Some will stand out from the rest at some point.
Though about the wyvern. It's not just time, it's the effort itself. Like there is a reason why wolves aren't common pets as they are difficult to train in general. Not impossible but very hard. THink how hard it would be trying to train a large reptile that is five times your weight, can fly and can tear you limb from limb within a few seconds.
I did however hint that it wasn't impossible and that there were a few that have broken Wyverns to be steeds.
I'll make a very rare exception for this because you give a good reason why he has the Wyvern and the fact that he is so old he would have enough training and time to be able to break a Wyvern into loyalty.