You want to use fighters to
ram planets? You'd better (didn't think of that earlier) stick a few Cluster Bombs on 'em, then you can actually
use the same fighters in more than one battle!
No, but really, I mean it. Try it out with Cluster Bombs and see if that'll work.
And that's only using
fighters. If you were to take a Baseship (assuming you're playing stock), you could stick an entire battery of Planetary Napalm Bombs on it and glass a small to medium sized colony in a single volley.