You can tank them that early, but it's going to suck terribly: you simply take more damage (since gear sub-30 doesn't have appreciable secondary stats on it and that's what Shadows live off of; at low levels, mitigation has more to do with your armor rating/DR than anything else, and Shadow DR is going to have you taking a *lot* more damage than heavy armor will), require more healing (since you don't get appreciable self heals to counteract your higher damage taken til you hit your late-30s), and can't generate threat effectively (since your only AoE is Force Breach/Discharge on a 15 sec CD and it deals terrible damage). I tanked that early on both my Shadow and my Sin while I was leveling, but it was definitely rougher than it was for the other two tank classes I leveled. You can do it, but expect the pain to come rolling in.
Thanks for the tips Kitru. that confirms what I was thinking.
If I could ask you a question Kitru what are the defense stats, and stat weights I should be looking for?