In NXT and Bitcoin the best way to handle customer deposits is to generate unique addresses for them. This increases the size of the database. In Simcoin transactions will be done similar to Ripple - with a short text field where you can put some special code to link this deposit to your account. Or you can register with your address as a username, which is possible because they are so short and mnemonic.
It increases the size of the database anyway. If you have data representing that information, you have to store that data. It is as simple as that. You cannot have information without it corresponding data.
Not as much as a new account. Simple numerical code in a text field can be, say, 8 bytes. Public key alone is 32!
It will inevitably increase bandwidth usage. Question is: who much is acceptable?
It won't. Overall network bandwidth will be less than NXT or Bitcoin.