If you pull a Chromium (or Bronzium) and a full toon drops that you already have, it's converted to shards, the rate at which it's converted isn't random, it's based on the star level of the toon when it's unlocked.
If a toon unlocks at 1*, you get 7 shards if you already have it.
I believe it's 12 for a 2* toon but don't quote me on that.
It's 25 for toons that unlock at 3* (I had Lobot drop from a Bronzium and that's what I got)
I believe it's 32 for 4* toons.
Maul is Chromium only, at least for now so keep that in mind if you're investing gear and mats into him, you're going to see diminishing returns as you level up since as a squishy 4* toon, he's going to become obsolete as you start to face the higher level opponents.