In the last few years selection for all fleets and sub-companies were done by interpersonal and decision on which fleet or company to start was based on demand taking the wishes of the applicant into consideration. Of course if you had a rating and experience chances were high to end up on the same type again, simply to save some money.
At the moment there is not much cross-training, only a few captains on the txl base were allowed to change from boeing to airbus.
It is not completely clear at the moment how the different fleets will change over the next year and a lot will depend on the general economy im sure. The boeing fleet will receive 26 new aircraft until the end of 2010 and i dont know how many new airbuses will arrive in that time. At the moment no expansion is planned and new aircraft will only replace old ones. On the boeing fleet that meens better fleet standardization, cat IIIb availability and newer avionic features like GLS and IAN approach capability, oh, and of course the short field performance kit for all new ones too. Dunno much about the bus as im not on that fleet.