I don't know what is popular in competitive, but any special can combo together.
The first one I personally discovered was echo-locator and inkzooka.
EDIT: Ooooooh wait I misread, speacial compositions, like team composition, but specifically to pecials xD
I haven't seen it personally, but the stream done on the Battlefy turf war tournament they talked about quad inkstrike comps being popular in japan turf war tournaments.
I know that those multi-kraken comps are mostly used in Tower Control, possibly Rainmaker, but not in splat zones.
Also, double bomb rush teams are quite effective in SZ and somewhat RM