It will be easy to find a group as healer or tank. Isn't true for dps. You can change your role in some raids eventually - from offtank to dps and opposite. But usually you play one role per run. If you started as healer don't try to tank / dps without circumstances lol
With the start of raiding it will be hard to occupy the spot as tank as well.
You shouldn't have troubles at the current state of server - whether tanking or healing. These talents are not so good if you deal damage. You will be fine in 5ppl instances and Blacrock Spire probably.
Don't mix the items if you want to pve. Follow canons, use standart gear. Usually you roll for items that fit your role in group. This is your absolute right. Also you can ask for rolling of items for your offspec if nobody need it. There is a high chance people understand yu and give you agreement.
Things become harder when we talk about pvp. I believe you should try all options by yourself and make your own decision after that, the decision that is based on your personal preferences in playstyle.
In my opinion hybrid playstyle should be based mostly on high survivability, decent dps in melee and big mana pool. Someewhere between these three edges you can find the biggest efficiency of a feral in pvp. It's like enigma - you need to find such combination of gear that proviedes you better way. Also there is no ultimate combination - you want to have more armor against rogues, warriors and hunters; - you want to have more stamina against mages. Spirit for battlegrounds, intellect for duels. And so on.
I don't recommend to avoid bonus from 4 pieces of your pvp set. The gloves from pvp set gives bonus to stealth detection. Somehow you need to gather 5% of hit chance.
There are few interesting items:
Tome of KnowledgeTimeworn MaceMask of the UnforgivenRing of Protection - 300 armor atm
Death's Clutch - no agility atm
Songbird BlouseThey can boost your hybrid attributes very well. You can find more of them...
Can't say about later patches