I am new to the game and recently tried out crawl mode for the first time. I survived a decent number of rounds but realized that Chameleons seemed never to appear, neither among the randomly-selected units during the first 12 rounds of the crawl nor within the unit offer between fights. I was curious about this, so I started and immediately forfeited a number of runs (probably 20-30) to see what the preliminary unit selections would be, and I noticed the same thing: no Chameleons, despite all (I think?) other units, plus shapeshifters, appearing among these selections.
Guessing this omission of the Chameleon is for balance reasons: perhaps Chameleon would go against the recruiting spirit of crawl mode since it would be effective with such a huge range of other units and also would make the scrolls go haywire. Just thought this observation might be of interest to others.