I've got a pally, priest, mage, warlock and hunter team to 70. I regret not doing 5 shammies back then but they are up to 44 now.

Don't want to dash your hopes but be prepared to spend lots and lots and lots of time doing macros and learning classes. Every time you level and get new abilities you need to redo all your macros. You also don't have the same burst DPS as 5 DPS classes and you don't have the healing power of 5 healing classes. You need to have very good healing skills. The pally tanking is pretty trivial, you will struggle to pull aggro off the pally unless you fight something much higher level where you will get healing aggro. ALso be prepared to write macros for certain boss encounters to deal with adds etc. I had a few dedicated buttons setup, tank only, tank/direct dmg, tank/boss (dots etc), burn it down now, debuff and healing for each different character. Also made dispell/decurse/cleanse buttons for every character.

On the other hand, if you like a challenge then you can have a lot of fun with it.

Are you RAFing, boosting or leveling them old school style?