Q1: Do all A340 SO's fly the A330 as well?
All Airbus S/O's start off on the A340. Once CCQ'd (after 12mths) you are rostered for both A340/A330.
Q2: What options are available in terms of bidding on/off aircraft types?
Upon upgrade (S/O to JFO or SFO to Capt) you have a chance of a type swap.
Another way to swap types is to bid for a base that operates a different type. You must use your joker however of which you have one only for your entire CX career.