Why so many aircraft?
My guess is that there is demand on the domestic market and therefore more aircraft are required. Specially since the OSL contract.
Why so many aircraft types?
There isn't that many...Dash8, F100, B757/B767 (i wouldn't really count the E-jets).
Why can't they have 2 757's instead of 1 767 and 1 757?
Because they require the 767 for its freight capacity.