Missiles are... not very big, I don't think your idea makes sense unless you are also proposing some sort of colonist compression technology.
Though it does bring up the idea of automated colony ships, like one shot vessels that bring colonists to a planet and then are converted into infrastructure/installations or something, which is an idea I could get behind.
As for terraforming missiles I sort of like the idea of loading up gases into missile and firing them at the planet you want to change from billions of km away, it seems like it would be incredibly inefficient though, and also a good way to use terraforming to kill populations without harming installations. So I think maybe it won't be happening sadly.