Can a Captain Bid on a First Officer Slot
 

Ok - this question might sound stupid but here's the scenario I am considering:

Say you have been with an airline for a no or years and made it as a FO on a 747. Then you upgrade to a captain but on a 737.
Now can you bid on both (737 and 747) or are you now entitled to fly only as a captain.

So getting back to the original question - can you bid on some routes as a Captain and some as a FO.

No. Once you've been retrained on another fleet, the company will want to get value out of you on that fleet. So generally, unless the company's requirements change, you would stay in that seat on that fleet for a couple of years. It would not be a good idea to keep changing around, either.