Base allocation is done according to seniority and is now common to both fleet.
A new 737 base in an already existing 75/76 one means that existing FOs and Captains will be able to bid for it. My guess is that should this happen, it will be very popular for FOs wanting to upgrade to the LHS but didn't fancy moving to DSA.
Still once you have joined you will be on the seniority list and therefore allowed to bid for it (from day one).
As the situation is rather fluid, who knows how long it would take you to get what you want... Not that long I should think.
No 732s at Tfly wiz so you are safe
... As MAX said, you get a type rating from the 300 to the 900. The company likes the flexibility you get when we are all type rated on all types. Makes rostering lot more efficient.