At the moment, you cant manually select the AI enemy fleets. However, several mods have work-arounds for this via external methods.
As for more randomly selected fleets, Legacy is doing what its programmed to do. Contrary to belief (Legacy only picks the most expensive ships it can get its hands on), Legacy's ship select process for the AI is very clever.. The AI selects ships that provide the best payload opportunities within a price range. If you change your ships and AI enough and you can get more randomized fleets.