Hi Jana,
1. Yes, you need to do, otherwise you will face performance problems .
2. Declare the object which was occupying more space(Article or doc number). i think doc number may take more space. pls check again, SAP_INFOCUBE_DESGINS.
Declare the required object as line item. and keep other objects into new dim or others existing one. if you have less than 13 dim, you can create new one.
Thanks