All ships should be able to be assigned to "protect trade."
They already can. As long as there's at least one light ship, you're free to assign as many other ships to that stack as you want. It's just not usually useful to do so.
Transports should have the best rating for producing income when protecting trade -- BUT -- if they are protecting trade alone, without any other form of escort -- they should not prevent the spawning of pirates in the zone. They might be able to fight off the pirates, or then again, they might be sunk or even captured.
So, basically dedicated transports go away, and are replaced by light ships being able to transport troops? Why? This doesn't really seem to add anything to the game, if anything it removes interesting decisions from the player, since they no longer have to decide how many transports they need. Do they need to be able to move their entire army? Or just a small force to protect islands?
Light ships should have the next-best rating for protecting trade income.
Galleys should have the third-best rating for trade income, but their rating might be doubled, thus better than light ships, in inland seas.
Heavy ships should have the worst rating for income.
It sounds like you're trying to give the player interesting choices for which ships they want to build, which is fine, but you're missing the fact that it's already an interesting decision for which ships you want to build. All these changes really do is make things needlessly complicated.
If Paradox really wanted to hit one out of the ballpark, just as they are doing blockade values based on how many ships you have blockading, you could do pirate suppression based on the total "anti-piracy" values of your patrol, vs. the "lucrativeness" of the route -- which would be attracting pirates. Again, transports would get you zero anti-piracy value. Light ships would get the best. But a fleet of galleons? They might do quite swell to keep pirates away from your trade routes too.
Dear god no. First off, protecting merchant ships from pirates is exactly what light ships are doing when they protect trade. I mean, it's in the name and everything. Adding a "pirate suppression" value to the node that you have to make sure you meet just adds an absurd amount of busywork and micromanagement as the player need to keep checking to make sure that his trade fleets have enough combat vessels as the trade value moves up and down and other countries add or remove their own ships, changing how much suppression is there.
