Hello,
With the Ruby Knight gate having Perimeter Prominade II at depth 19, I thought I'd try my luck at finding a Sloom. As of yet, I've not found one, and I've done quite a few box openings.
However, I found something stranger..
http://wiki.spiralknights.com/Exploration#Monster_Cages
As you can see, Mecha Knights are not an enemy that you can find here, but I've found 2 now. It's not a good sample by any means, but I've found 2, whereas so far I've found 0 zombies, lichens, silkwings, gremlin scorchers, demos and understandably sloom and love puppies.