I agree, a tank would be nice in this group, simply because Templar and Mystic just don't have very much DPS till later when they can spec it with abilities.
The group you mention will never die, you just won't be able to kill much stuff fast.
That said, you are a 'group looking for 3 more, need healer' wet dream
P.S. When choosing a tank...Pally's here are nothing like Pally's there. Here they have some healing ability, but nothing like a WoW pally. Shaman, Druid and Priest (cleric) here, are nothing like the WoW versions, either...although all 3 classes are fun to play.