The thing about preemies is not necessarily their weight, but their positioning. A preemie can get into the curled-up/chin-to-chest position really easily and that impacts their ability to breathe.
That said, I taught my SIL how to use the Moby when my nephew was born; he was a little more than 5lbs at the time. I emphasized the importance of pulling past the stretch AND ensuring his airway was open. (It also helped a bit for his reflux.)
Based on my, albeit limited, experience, I think that a wrap would probably be the best option to use with a new squish that tiny because it's quite adjustable; I would choose something like a Wrapsody Hybrid over a Moby or woven, due to the width of the fabric and the one-size feel rather than trying to figure out sizing (like with a woven) and pulling against stretch (like with a Moby.)
The biggest issue you'll have with the K'tan is sizing - most folks order theirs too large - and the Gemini, even with a rolled blanket under Baby's bum, might be too long for a baby so small.
For more info, and a few links to videos, do check out Preemies and Babywearing at Babywearing International. Hope this helps!
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle