[Gryphin]

"3) Would there be any good reason to use a non-colonizer hull to build a colonizer?"

1) Speed. You can go 1 sector faster in a Non Colony ship hull. It is a trade off between a 18%? Increase in speed and the cost of the hull. How much will you save in Maintenance cost by getting there 1 or 2 turns faster. How much will you get back by getting a facility in service 1 or 2 turns sooner.
2) Cargo: You can put more cargo bays in a Light Crusier size hull than a colonizer hull. This way you can carry more Population and if going to the edge of your frontier you could even carry a weapons platform or other unit.
3) Supplies: You can send a Colonizer further. I could see this useful if you have Ancient Race and can spot good planets….