Okay, well, I have something of a linguistics background, but I am not an expert on Asian languages. It seems to me, however, that if the words of the pedestrian are true ("Those Tsoo make the Hmong community look bad."), then the case is solved, the Tsoo are Hmong.
On the other hand, the Tsoo seem to have dress styles that could range from Chinese to Japanese. But the word Tsoo, while certainly Chinese, is not Japanese. Of course, we must consider that "tsoo" is romanized, and there may be several spellings of a Chinese word, depending on which system of orthography one chooses. Doing a quick search on the Internet does reveal that the word "tsoo" is the name of a language, and the people that speak it, in Taiwan. It is also the name of a writer, Chiachin Tsoo, from Taiwan. Furthermore, any search including "tsoo" and "taiwan" or "chinese" yields a multitude of Chinese/Taiwanese examples of the word "Tsoo" in other words. Chinese culture is quite dominant in Taiwan, and it is likely that the Tsoo language is a derivation of Chinese and not Japanese--which is apart of a different language family altogether.
Now to why it isn't Japanese. Simple. The "oo" part is not found in any Japanese romanization. If we assume that "oo" is like Korean "oo," then it would be pronounced in English "soon" or "broom." But since the Tsoo are in Taiwan, we might assume it sounds more like Chinese.
And in Chinese, at least in Mandarin (which is the official language in Taiwan), the closest vowel sound is actually a diphthong (that is, two vowels which sound like one vowel like in English "high," where "igh" is actually "a" and "I" according to the IPA). Thus, the "oo" seems to be an alternate spelling of "Tsu," which we find, is a very Chinese spelling (and while also Japanese, we can discount that origin since no Japanese orthography claims "oo" though many Chinese languages/dialects do); and it is pronounced more or less like "ooh" in English but with a stronger labialized offglide (that is, the second vowel has a more "w" quality to it).
In Mandarin, the Wade-Giles system uses "ts" to describe a similar sound found in English "cats"--referring to the "ts" cluster as the final sound. In pinyin, the official romanization system, "ts" is replaced with "z." Honestly, it doesn't get any simpler than that.
Chinese is a tone language, and we can assume, quite safely that Tsoo would be tonic as well. If Tsoo in COH is Taiwanese or Tsoo, then their name has a tone to it. I cannot conjecture as to what tone it is since there is no indication of it in the spelling.