Can't see any reason why you wouldn't be able to. It's not even optional- so long as a Gemini is alive, you HAVE to put wounds that Celestine suffers on it.
Wasn't Celestine S4/T4 previously? I don't remember offhand, but I vaguely recall she was.
Cawl and Celestine are both pretty tough to kill, but not outside of what current armies can manage. A volley or Grav, or enough S6, or whatever will still drag them down. Being able to sub them in for other detachments is cute, but probably won't get used a lot. Cawl's bonus canticles are good, but there's a lot of good options for canticles already and the text would seem to indicate you can't pop two at once (it's just three extra options in addition to the others you have.) The invuln is potentially pretty good as a way to fight armies with extensive Ignores Cover, but since it only goes to a 4++ it's not gonna stop a huge amount of firepower.
The "super ultra mega detachments" look kinda bad. They all have really high minimum requirements and don't really open up many options that you couldn't already do. Interesting ideas, but I doubt they're gonna pull anyone away from available builds, especially because many of them are actually pretty restrictive with what you can take.
Relics are interesting. We may see some use of them, since there's a couple gems in there, but the requirements to access them are a bit strict.