Transports

They have me confused. I know some units may purchase transports for a set number of points each but I also know some tanks and such can hold troops. I always get confused at the start of a battle as to whether or not I can start with a certain set of troops mounted in a certain transport that isn't listed in the codex as "their" transport. Some help please? To use a specific example, Battle Sisters transport is a Rhino. But if I wanted to throw them in something completely different, like an Immolator or something, is it a legal move? I know using Battle Sisters, fitting 10 models in a six model transport is illegal, but hypothetically....

The only difference between a transport and dedicated transport is that if you buy a dedicated transport only the squad you bought it for can start the game in it where as a regular transport can start with anyone. For example:

An Eldar Aspect warrior squad can buy a Wave Serpent as a dedicated transport. I can also buy a Falcon as it's own Heavy Support choice. At the start of the game only the Aspect Warriors I bought the Wave Serpent for can start inside the Wave Serpent where as anyone can start inside the Falcon.

P.S. Once the game has started anyone can enter any empty transport.