> 穷 (qiong4) and 丑 (chou3) combine to make "qiou",

穷 is actually qiong2, i.e. 2nd tone

> though I can't even guess which tone it would be.

Because 没 (mei2) and 有 (you3) combine to make 冇 (mao3), I'm guessing 穷 (qiong2) and 丑 (chou3) combine to make qiou3, i.e. 3rd tone.