As with many technical discussions this one quickly went beyond the scope of the interwebs. Tuna, go to aerospace design school for a full reply to your questions, or at least buy the books listed below. You might try Eng-Tips Forums for aerospace engineers because this is mostly a pilot forum. The big picture on how to choose engines, how many to choose, where to put them is complicated preliminary design topic which includes many factors such as:
Number of engines to be used
1. power required and currently available engines on the market
2. relationship-critical factors such as climb requirements, field length, and engine failures
3. other safety considerations (ETOPS etc.)
4. cost- exponential cost increase with more engines
5. fuel available (biofuel, JP-8 etc.)
6. customer specs
Mounting factors
1. effect from engine loss on stability and control, thrust line, etc.
2. weight and balance effects
3. weight
4. engine efficiency effects due to location
5. vibration and noise
6. maintenance access
7. aerodynamic effects on aircraft from engine exhaust and mounting hardware
8. engine clearance
9. attachment points
10. thrust reversing requirements
11. engine driven accessory requirements
12. fire requirements
13. FOD exposure
A good basic design series on this is
Airplane Design, Jan Roskam, Parts I-VIII. DAR Corporation ($350). Hope this helps.