"/summon Mew ~0 ~1 ~0 {Growth:1,BossMode:1s}"
the system will fail with the message "Unable to summon object." The whole point of doing this is so that I can create custom spawners that are hidden, that create pixelmon boss that are the right size for the room they are spawning in, right boss rating, and right location. I'd even love it if the pixelmon had an AI that would stand in one place as a boss until the player reaches a certain distance... but that's an entirely different subject. Right now the /pokespawn command is extremely limited as is the spawner, but I'd like to do more.