I would stick with a long cage unit of your choice it will work with all the cassettes you mentioned 12-25 11-28 12-24 11-34 and so on. Also a 9 spd rear derailuer will work with 9spd or 10 spd as the indexing is done in the STI shifters shifters not the derailuer. A good Shimmano XT will work great. The Ultegra or 105 long cage units are great to but this will limit you to 11-28 max on your rear cassette but work just fine for all the other cassette opsions mentioned. Again in my opinion by staying long cage you can use all the cassette opsions just encase you want that 11-34 back on your tandem.

On the front derailuer it does have to be 9 or 10 spd specific to lift correctly due to chain width the internal part of the chain is the same that is why a 9 spd rear derailuer will work just fine again due to the shift indexing being done with the STI road shifters.

10spd cassette with 9 spd shimmano LX long cage rear derailuer on bikes below. They works great on my Co Motion NorWester Tour road bike and the same set up is on my Co Motion Tandem in the pic's below, works great shifts great.

Ride Safe,
Bill G