About "small" navies:
For Prussia and Austria, I’ll just use a generic Navy (as all the minor faction). Almost no preplaced units except some sloop or brigs, but if you want to play one of these factions, you should be able to recruit something.
I think a 74 max is more than sufficient as generic unit. And with a low limit.
Perhaps 2x74, 3 x 64 and 3x50, then only frigates.
About the recruitement of navies.
The recruitment of the navy is limited to the national regions. I.e.: British ships only recruitable in the 4 British region.
Then, if the French capture a British region, they could built British ships instead of French ones there.
And so if France capture a Prussian region, they won’t be able to build something better than 74, even if France have the resources and knowledge to built a 122
My reasoning here is that to develop the naval yard, you’d need also some knowledge and local shipbuilding tradition, and I think it’s difficult to acquire in the 10 years of the campaign.
So if you play as Prussia and want one day to invade Britain, then you’d better try to capture Denmark or Holland, to use their shipyard to build a stronger navy.
Not only because they will allow bigger ships, but also because since the ships are region limited, and have a unit limit, capturing “naval regions” will increase the unit limit you can have.
And for Russia, I have separated Baltic Sea / Black Sea, with different ship classes, each with its own limit.
Last, the national navy can have several classes for the same rate of ship (like Britain has several 74-guns classes). But for "occupied navy", there will be only one class of each.