Starting with a single control plane node in a cluster, should it be ok to add additional control plane nodes in parallel? I currently do it sequentially and don't know if switching to parallel add would cause problems with etcd or something else

shouldn't. I spin up 7 instances at once in AWS, and they all start at the same time, as long as they are configured properly ( $leader with no server: token, others DO have server: token) they'll load in fine

you can try to add as many servers as you like at once, but it will not actually work immediately. You can only add one member at a time to the etcd cluster, so if there is one in the process of joining, the others will fail and retry.