Thomas SG, I could transport a rifleman (and lateron also a settler) with a caravel, but the hover text says it can only transport scouts, Great Person etc. So, there is either a bug or the hover text is wrong.
What cargo concept are using? Standard or Advanced Cargo? If you are using the advanced system everything is fine. Have a look at the hover text. A caravel can carry 1 unit of the domain land and 1 unit of the special unit type people. So a standard caravel can carry for example one rifleman and one missionary at the same time.
The carack is better because there is no limitation for the second unit. It has two cargos of domain land.
If you are playing with the standard system it's bug.
Here are some pictures to show you. I think you will like them. I've added three secret hints what I'm doing at the moment.