With my first 2, I started part-time ECing around 7 mos -- when they were strong sitters. We were never diaper free, but we did get quite a few catches. Overall a positive experience.
I find the idea of ECing a baby who can't hold up their head a bit overwhelming. At the same time, they're usually pretty obvious about going, so it seems like an easy time to catch rather than leave it in a diaper. Because of catching so many poops with babies, I find it very weird to see parents of toddlers watch them go in their diaper, knowing exactly what's happening and waiting for them to be done. But that's the same thing I've been doing with a newborn.
I'd like to hear your experiences about ECing a tiny one.