It'll be crammed full of stuff, that's the nature of ships like that. Emergency equipment, pressure suits when not in use, backup food supplies, and so on. Also, delivering cargo to the station currently runs at a cost of around $50 per gram, give or take, so they're going to make use of every launch they can to bring things to the station.

Do you know what the cost per gram will be with reusable rockets?

That depends on who the customer is. With NASA there are lots of bureaucratic hoops to go through which raise costs substantially.

Otherwise it depends on how many flights you can get out of the capsule and first stage and how expensive operations are. Assuming that operational overhead is minimal, and assuming that you could get, say, 5-10 flights out of a Falcon 9 first stage and a Dragon v2, that leaves you with a rough cost per flight of maybe as low as $20 million for a manned Dragon and maybe less for an unmanned Dragon. Unfortunately the cargo that NASA tends to send to the ISS is often quite bulky so the cargo Dragon tends to be more volume constrained than weight constrained. Potentially the Dragon could take up about 3 tonnes of cargo, though sometimes they only manage around 2 tonnes due to volume, but that leaves a cost of around $7 per gram, which is remarkably better but still extremely expensive. And note that even though a reusable Falcon 9 could potentially deliver payload to LEO at $1/gram ($1k/kg) or less the extra overhead of delivering actual, packaged cargo in a pressurized environment to a station adds significant cost.

To get back to the other point, even at $7k/kg there is substantial incentive to pack every vessel headed to the space with as much cargo as is reasonably feasible.

Why would putting cargo into the crew compartment be unsafe? on the way up, there is no danger of the cargo falling on them... quite the contrary...

Because fundamentally, if a craft is safe to send humans to orbit you want to send humans - not cargo, which is cheaper to send with a craft which is less safe.

You are correct to say that using a human-rated system to launch cargo is usually much more expensive than necessary, as was true of the space shuttle. This does not mean that the cargo causes any safety problems.

Indirectly, as you have to launch another spacecraft for the same number of people to get to orbit.

The least they will need to stuff in there are the life support systems (food,air,etc) to deal with delays/emergencies