Modding in a Patrol Ship

make them a ship class. smaller tonnage than a frigate?

my bad. didn't realize you couldn't create new cap. ships.

scale all the cap ships existing down? so frigate becomes patrol ship, destroyer is frigate... leave out light cruiser size?

I would have to agree with Cerberus.
Since there are no Heavy Cruisers or Battle Cruisers in SE V, I see no problem with tossing out the Light Cruiser in favor of making a ship smaller than a Frigate.

I would probably call the new small ship a Corvette, or an Escort (which was in SE IV).

The only real problem of making a yet smaller ship is the available size, as even a Frigate can be a tight fit.

I know it is possible to make it so a ship requires multiple engines per move, but I don't know if it could be possible to have a small ship get multiple moves per engine.
One work around might be to reduce the max engines, give it a minimal engine requirement, on top of giving the hull a base movement bonus.

For example, reduce max engines to 8, give it a requirement of 4 engines, but give it +6 to movement to represent being a ship built for speed, and be able to outrun frigates but not fighters.

Ok yes you could toss out the light cruiser but you wouldnt have to. Just copy the frigate and change the name, size and stats.

Yes you can make a ship get multiple movement from an engine just set it to .5 engines per move and you will get 2 movement per engine.
make sure you set the number of engines accordingly as they will get double movement.

One thing to remember is if you don't edit the AI then the computer won't build them.