as with any group, it entirely depends on what you want to accomplish. I always advise folks to play what they like, as getting some enjoyment along the road will keep you playing long after X thing made it hard.
I have played every class in EQ, and have a top level/nearly maxed of every lass except BL. They all have their positive and negative aspects. Personally with your combo, I prefer a Shaman/SK, since you can always pop a cleric merc. But a cleric toon bring a LOT more to the tank and makes it well worth the effort. I would think, since you have the meatshield, and something to keep the meatshield up, you would shoot for either a DPS or Utility class... depending on your preferences.
Depending on how far up the level chain you want to go here, mercs SUCK as you get higher in level, even with the gear you can buy for them. They are great, if you can not fill the rest of the group or need X class in a pinch... but they flat do not compare to a (relatively speaking) live toon. That being said, DPS/tank mercs are decent enough at what they do but deteriorate someplace in the 95-100 range, and get worse 100+
That being said, you an go with a utility toon and 2-3 DPS mercs for decent enough damage, if you are not planning on topping out in levels. Otherwise I would recommend a DPS class of some sort for your third slot.
And again, with that on the table... if you do not plan to level past 100, I would make some FTP accounts and level them up. As Raz can attest, if you max the level on your FTP accounts, then get them a krono or pay for a 1 month sub, the autogrant will pump up their AA ( plus what ever you gain for that 1 month) then you end up with a FTP account thats better then a merc... especially since you can better tune it's behavior!