How many transports?

[Lerra]

I'd say by RAW you are allowed to take both a rhino/razorback and a drop pod. I don't see anything that would exclude that, and it doesn't seem game-breaking or anything like that. I'd allow that if you played against me.

Keep in mind that the same argument works for Blood Angels, though, and their troops can take dedicated land raiders! I'd laugh if I saw a list with 1 HQ, 2 troops, and 7 land raiders as dedicated transports, and then I'd whack the BA player over the head. As far as I remember, the wording in the BA codex is that assault marines "may select any dedicated transport".

[Lord Azaghul]

No, as you do not split into combat squads until you deploy the unit.
As the DP is in reserve , you can either put the whole squad or none of the squad.

Just so you know I don't seriously believe this is ever a valid option!

[lobster-overlord]

Yes, the BA assault marines can take a Land Raider, and pay 35 points (I think) less for it. This is the Discount Deep Striking Assault Land Raider that contributes to BA being Assault Broken lists.