Of course AA has an interline agreement with THY! It sounds like the THY ticket is using an unpublished fare, hence Travelocity cannot ticket it with AA (and you are right to be concerned about your excess baggage fees with THY, they are likely to be higher than your ticket price!). Check aa.com and see how that fare compares with what you have; if much higher, it confirms that you got a consolidator fare and not a published fare. Your options are limited if you need the baggage allowance.