Hi!
Welcome to the board Baduin.
Here's some fluff from the real old days of epic....
The crusader and castellan are "heavy knights" sporting heavy weapons. They are slower, less apt for physical close combat than the standard paladin chassis, but to compensate have higher armor and longer ranged weapons. They serve a support function in the knight households.
In the old days they were called "wardens". These were senior knights whom age had taken hold and werent as good as younger knights for the quick "hack and slash" of knight-to-knight combat. However their experience and dogged determination were harnessed by the household by using old knight chassis that were heavily armored, slow, but armed with the biggest guns the household could provide. They mainly serve guardian duties of the main household fortresses, but were used on the battlefield as heavy support.
The old warden "morphed" in the crusader and castellan in SM2/TL, but thats the background concept behind them.
A veteran princeps is someone with battle experience. If you used the old AT experience charts a princeps with a certain amount of kills (very high indeed since it was the highest rank under Ace). SO from a background perspective a veteran princeps is someone with a good amount of battle experience against other titans and has many "kills" on his banner.
Legate, under epic at least, I have never heard of this term. It may apply to 40k, but no epic equivalent has exsisted officially. Although some conversion may exsist somewhere on the web.
Primarch