Yes ferals can tank in 25man. In fact taking Gruul's Lair as an example you would almost definitely need 1 or 2. Also handy in The Eye where you will need 4 tanks for Reaver, but don't really need 4 for trash.
Unless your guild plans on swapping people in and out a lot, I'd say you definitely need a feral or 2.
At the moment, my guild always runs Kara farms with 1 Warr and 1 Feral tank. Our trips into 25mans usually have 2 ferals.
Just make sure you are geared up well, Heroics/Kara/Tier etc and you should be ok as a tank/dps hybrid in 25mans.
There are plenty of threads on this board about gearing. You can also check out emeralds list if you need to.
Don't know what experience you have or where you guild is right now, but try to get lots of time in Kara. Learn how to tank the trash and bosses in there.
Even as a feral you should be able to main tank every boss in Kara. This will serve you well to expand your skills in 25 mans.