Is there a way to make large Auroran Warships come from a planet after a major string has been completed? I keep changing the defense dude to large ships for a couple of planets but when I demand tribute, abominations only come out. However, whenever I demand tribute before a major string has started, large ships come out. Is there a way to fix this?

When you complete a string, large ships will no longer appear anywhere in the game (except for you to buy). You can change this by messing with the AppearOn fields for each ship in question. I can't say exactly what to do but basically there'll be a !bxxx in there somewhere that you can remove.