I might drop the beginning part, even, using only the end. "Somnaissi" has (at least the way I'd pronounce it) three syllables, flowing a lot better. If wanting to obfuscate the origin, turn the esses into sh, ending with somnai + shishi -> Somnaishi.
I was additionally thinking of other "organized crime", but nationalist organizations go to the other end of the scale to Templis & CPD (Academic Carelia Society, for example, supported uprisings in and sent military expeditions into soviet part of Carelia). However, the Finnish word for "society (club)" is "seura". Doesn't work together with "somnai" as "sissi" does (at least in my opinion).
Finally, for completeness, I'll note that I forgot to mention probhibition-era smugglers from my list of criminal gangs. The term doesn't work very well here, either: the slang term for "(illegal/questionable goods) dealer" is "trokari", from verb "trokata" (sing. trokaan / trokaat / trokaa, pl. trokaamme, trokaatte, trokaavat).