I'm looking at some air warfare ships for Thailand, as opposed to the current crop of general purpose cruisers and ASW oriented DDs. I'm also looking at using Diesel propulsion on some ships. Are there any particular restrictions that I need to know about?
You might want to look at the Kriegsmarine's frigate designs - the Klagenfurts are more general purpose while the Manchings are dedicated AAW.
As far as diesel engines go, Springsharp does not seem to distinguish much between diesels and turbines in ship design. Game wise, I don't believe that we have formal restrictions.