They are getting 5 more 190's next year - the odds of you being on this airplane for a long period of time are as good as you bidding off of it.
To get "stuck" on the E190 would require an extraordinary stroke of bad luck, it's nowhere near 50/50 odds. Everybody who's been on property over a year can hold something else and I don't see that changing. Seniority wise most new hires can hold Airbus right off the bat on their first bid. People stay on the E190 only because either they choose to for schedule or because they are assigned it as a new hire (and become locked for 1 year at a pay rate which is higher than the Airbus... don't throw me in that briar patch!)
5 new airplanes equals 25 more FO's which is a drop in the bucket. Hiring is projected at 250+ per year going forward due to retirements, American is even more. Would a merged airline result in some consolidation of routes and a slowing of hiring? possibly. But it would have to be extreme to offset that level of retirements because without hiring the company would almost completely evaporate in 15 years.
Now one thing that could happen, in a merger scenario, is that the company gets their "B scale" wages on the E190 and also gets the contractual right to replace MD80's and such with E190's, greatly expanding the fleet. Something to watch out for during contract negotiation time. Of course I've also heard that supposedly the E190 rates will go up significantly to around JB levels so as to not be nearly so heinous, but still nowhere near the proposed AB/B737 rates.
But if you are sitting on the fence right now deciding whether or not to apply I wouldn't let any of this speculation be the deciding factor. If you think you want to work here, do it sooner rather than later and chances are you'll be sitting long-call reserve in an Airbus well before any merger even starts to affect anything operationally.
Mach-tuck: The odds of a merger are no sure thing but right now the general feeling among most of the people I've talked to and most of the press articles is extremely likely.