They've never named a Sontaran with the rank of Captain.
Here's a list of all major Sontaran characters on Doctor Who and The Sarah Jane Adventures:
- "The Time Warrior" (1973/1974) had Commander Linx
- "The Sontaran Experiment" (1975) had Field Major Styre
- "The Invasion" of Time (1978) had Commander Stor
- "The Two Doctors" (1985) had Group Marshall Stike and Major Varl
- "The Sontaran Stratagem"/"The Poison Sky" (2008) had General Stall and his right hand man, Commander Skor
- "The End of Time" part two (2010) had Jask
- "The Pandorica Opens" (2010) had Commander Stark
- "A Good Man Goes to War" (2011) had Commander Strax
- "The Sarah Jane Adventures" episodes The Last Sontaran (2008) and The Enemy of the Bane (2008) both had Commander Kaagh.
There have been a number of other Sontarans in the novels, comics, etc., and a number of minor named characters on TV, but none of them were ever refered to as Captain.
It seems like the rank of Captain doesn't exist—or perhaps Commander is the Sontaran equivalent of the British Army's Captain. The people directly under Commanders are Lieutenants, and the people directly above Commanders are Majors (or Battle-Majors or Field Majors), so there's no room for a Captain.