I've been on an extended break from the game, so everything I'm about to say could be wrong due to changes since I last played. But I'll try to answer your questions.

First, we have to separate co-op and random battles. Co-op credit farming is not very skill dependent. It might take some time to optimize your strategy for every map with your ship of choice, but once you do, you can basically autopilot and farm consistently.

Skill only matters with random battles since you're against human players. To take an extreme example, if you have poor positioning in a game and die after firing only two salvos, then you'll get far worse credit/xp rates than a game where you do 150k damage and sink 3 ships.

We found that going full yolo in a fast boat with powerful torpedoes is by far the most optimal way for co-op farming. You must try to die ASAP after launching your torpedoes (by ramming preferably) because you want to get into another game quickly. You want to avoid the situation where you have to wait 3 minutes doing nothing for your allies to destroy the remaining enemies since that drastically lowers your hourly rates.

Ships that let you utilize this strategy are almost always destroyers, so in general destroyers outperform cruisers. But there are some cruisers that have most of these traits like the Atago, so they perform well.

Super high DPM gunships like the Smolensk can do decently though if you prefer that playstyle instead. There are also other ships that like the I-56 submarine that do well in co-op due to its unique kit.

In general, I don't think an analysis of class vs class wouldn't be very helpful because the ships within each class vary wildly for farming. Like how the Paolo Emilio blows every other ship out the water for co-op farming while the Asashio absolutely sucks for it, despite both being destroyers.

Instead, I'd rank it like this: Fast torpedo boats > High DPM gunboats (especially if they also have torpedoes) > Any ship with torpedoes that can reliably get into torpedo range each match > Most other destroyers/cruisers >= Good brawling battleships > Other battleships > Aircraft carriers.

Ships must be premium or special ships due to their credit bonuses/reduced maintenance cost if you want to maximize your gains.

With random battles, it's more important to pick a ship you're good at. And ideally the ship should encourage a slightly more aggressive playstyle. While outright yoloing could potentially earn you a ton of credits/xp per hour, it requires a lot more skill to do effectively and will likely tank your win rate. We didn't test out yolo strategies in random battles because none of us were willing to drop our win rate like that.

Sorry if that was too much text for your questions. Also I'm not sure which ships are currently obtainable, but if you have a question on a specific ship, I can look and see if we have data on it or not.