They will diversify. It is random which ship type. Shipping lines will also be randomly created. In my current game 10 years in, I have three lines, each with 1 colony ship and the original line has recently added 3 freighters.
It was rather annoying having to generate about 1000 infrastructure for mars myself. Plus the lines wouldn't deliver colonists if there wasn't room so they couldn't make any money. But then when there was room, they'd all go at once, so then I'd have unrest from overcrowding. So, I ended up having my colony ship taking colonists back to Earth.