I tried making the other ships just move randomly, but because you couldn't then predict their movement, they weren't any fun, just frustrating, so now they home in on the player. It's done by modifying their x and y velocities so they move quite nicely. Three hits destroys the ships, unlike the mines which are invunerable.
I wanted to get their animations out of step, so I added a CAnimation::Random() member that gets called when the various objects are set up at level load. It just calls CAnimation::Tick(float) a random number of times.
The other ships also only activate when they first appear on the screen so a well designed level can have them flying at you from all sorts of unexpected angles. Since your missiles are only horizontal, they would be hard to fight if they get above or beneath you but that might be cool as well.
I'm going to try and write a shortlist of features I want to add before I put a final game together. If anyone has any suggestions for additions to the list, they would be gratefully recieved and considered.