Standalone LXD servers can’t join a cluster without being wiped clean first, but that’s luckily not what you’re doing in this case.
In your case, you’re starting from a standalone LXD server which then gets turned into the initial cluster member and the cluster then gets grown to 10 servers total. That works perfectly fine.
You can either bootstrap that initial LXD server as a cluster right from the start (a single server cluster effectively) then deploy your MAAS container on it, deploy the other servers and then join those other servers into the cluster.
Or you can not even setup the initial LXD server as a cluster and configure it and use
lxc cluster enable later on. I’d personally recommend initializing it as a cluster right from the start as that avoids having LXD do the DB reconfig/re-shuffle later on.