Thanks, everyone! This has been very helpful. If anything it just reassures me that I'm not crazy to be waiting. My gut just tells me they're not ready yet, especially since they can't sit up on their own. My smaller one is, the ped & I believe, "younger" than the other one in many ways--and measured a month behind at birth. So in some ways her system is less developed, more like a 30 weeker.
I don't think I'll wait for teeth, because my older child didn't get any until 8 months! She did well on pureed veggies, so I was fine with that. I don't think the teeth do a lot to help them chew, anyway, but I know everyone has their own viewpoints & feeding methods.
Our 6 month visit is in a couple weeks, so I plan to talk to the ped about it. She knows I'm cautious about everything (vaxes, solids, etc) so I know she won't pressure me.