I'm at work so can't confirm for sure, but I don't remember having the meteorite guards available as EA R'Leyh, although I have used them as MA R'Leyh. The standard atlantian and merman slave troops for EA are morale 8 if I can remember correctly.
I did in fact merge the elephants with the shamblers later on and it's been one nasty trample machine.
I've never tried taking sloth as R'Leyh as of yet, but as I said I'm still pretty green. I could see it for MA R'Leyh, since I prefer the unarmored, and therefore less resource intensive, ilithids for the strat move of 2 over the 1 of the armored ones, but for EA R'Leyh I have been heavily reliant on indy troops (and summons) for ground pounders.
One advantage of the Shambler Thralls is only needing 1 resource, which would lend itself to some sloth. I'll have to try that sometime.