I find that Carrara's fire objects are somewhat limited; you're right, there's just the box, cylinder and sphere.  Not terribly useful if you want to set a car or bridge or house aflame.

I actually use a separate program to simulate fire when I need it.  Or if photorealism is necessary, I'll use an actual video source of flame with an alpha channel mapped onto a 2D plane inside the Carrara scene.

I havne't played with particles in Carrara much, but they can be very powerful in other programs, so I'd guess they have a lot of potential in C5P.

HTH.
Dex