Pug- sorcerer
Milamber- pug as a sorcerer turned Psionicist.... influences from Dark Sun a possibility as they merged magic and psionics to create the Dragon and the Avangion.
Kulgan- Wizard
Differentiating Low magic and High magic from what I remember of the books was.... Low magics allowed for level 1-3 spells, maybe 4th. high Magic has access to the higher levels but are triggered by some emotional response like almost death. To portray Pug/Milamber it would have to be ...
Pug starts off as a squire, so he is a fighter or warrior. At least level 1, then when he got apprenticed to Kulgan, he was still set in his fighter training that nothing was getting through, then during the attack that he had to defend the princess, the danger called forth his sorcerous powers. So now we have fighter level 1, sor 1. But the initial burst of the power was higher than what he was able to normally cast, so when Kulgan tried to teach, he can't do it. He needed to gain the experience. Later he gained enough levels to actually cast the spells without the books.
Id say that when he was captured during the rift war. He was fighter 2/ sor 3 around there. Then he was doing slave work, with a few climbing and such and maybe a fair share of fighting that wasn't shown in the books. so when he was eventually taken into the assembly he could be FT2/ Sor 4 or FT 2/ Sor5.
The assembly training was rigorous and required a mind wipe, or psychic surgery to lock memories. Hints of mental training here.
When he was trying to pass test he had to use his mind to open the door, any other way meant death.
So we now have high magic being having the properties of level 4 and higher but acts as psionics. If they had used the old d&D, the psionics rules weren't clear cut so using it as a reason for higher level spells was certainly a good way to go.
By the time he destroyed the arena, he was a highly skilled sorcerer. Most likely epic since meteor storm is a ninth level spell. So he was then FT 2/ Sor 19 or 20.
When he was helping Macross the Black , he could have gained a few more levels. At the end of the rift war, it's been known that he was travelling between the two worlds on his own will. Epic level feat "transverse dimension" or something like that could be created.
Can Midkemia translate over to d20. I think so.
It's all just a view of mechanics. Pug/milamber is a special case character. so was Thomas.
Thomas end result with Valheru items.- FT 25, with a Valheru Tabard. The Tabard granted great strength and merged Thomas with the Valheru that used to own the tabard. It is quite conceivable that the FT level could be higher. With spell resistance as well.
Anyway just some random thoughts. I hope this was an enjoyable read.