You can certainly play the healer at the same time as the tank in LFD levelling up. Keep in mind that no one but the tank should even take damage for the first 40 levels, and choose a healing class with a nice [@focus] rotation (I love figuring out healing rotations). I would recommend against monk because it is a pain to heal until level 30 or so, but anything else can be macroed to put out enough healing to keep a levelling tank up. Especially if both the tank and the healer have some heirlooms.

I wouldn't mix ranged and melee dps in the same 3 person team, no synergy and kind of annoying.

Keep in mind that you can't "blast" level your guild this way, as there is a limit on guild dungeons that give you credit a week, I think its 10? (might be 5). Probably want to be prepared to quest out the zones you need for loremaster if you really want to make a dent in the guild xp. (I've actually gotten most of level ten this week just with finishing a lower level zone and doing vashjir on one character).

Another humble suggestion, you might want to consider mixing a paladin tank with ranged dps and feral tank with melee. I find that a paladin works the smoothest because her pull spell is ranged anyway. I personally dislike charging in and breaking follow when doing trash in dungeons, but thats just me. In fact, for the vast majority of low level dungeons, you never need to break follow, cleaving bosses are pretty rare (and pretty puny).