Tyberius...
I will refrain from swearing, but the difference between battle, standard and light cruiser only applies to Imperial navy.
The power arc is far muddier when it comes to alien races, and using it to brand them is trying to make every race Imperial navy.
Everything is measured, clasified and named upon imperial standards in the 40k universe... and every fleet/race has it's own ship displacement clasification...
.....and there's no need to swear.. we are adults, (I Guess)
You will indeed swear reading this:
Tau are Copiers, that's what they do, they copy and adapt technologies to their own needs, always making them better, and more advanced. I must say, this is why they are so successful, and why they are always evolving..
Ok, lets say the Kor'Vattra fleet is made under imperial standards
Explorer= imperial Battleship
Hero= imperial cruiser or battlecruiser
merchant= a mix between a transport and a light cruiser (odd ship)
Then we must say the Kor'or'vesh fleet Is configured by Eldar standards. (just cause eldars did it first, or maybe a result of tau investigation on eldar ships)
Custodian= eldar battleship
protector= eldar cruiser
emissary= eldar light cruiser
Evidently we have another type of fleet, that surely can stand a fight by themselves, but my point was on this: It is a different fleet with different roles and features, so no ship in the kor'or'vesh fleet is a replacement for a kor'vattra ship.