1. Of course without flight intensity estimate it is difficult to propose something but at least as a starting point I would consider one crew dual type-rated and two crews rated only on one type each. This would provide necessary flexibility and keep crew maintenance costs at reasonable level. And I would be against 100% one type rated crews because it would create two different "teams" in one small company with very limited information exchange. In result you could get two mini-operators.
2. My company has a lot of operations away from base and we came to 3 weeks roster as an optimum.
3. I would certainly prefer Capt / Co-pilot crew as more stable and somehow safer. This is especially true in small company with limited number of pilots. Of course specific personalities can make a lot of difference either positive or negative.