Three of my hissers are apparently subadult (just after the molt they're 3cm or so long and >1.5cm across). Two are males, and the "horns" are only about 1-2mm out of the thorax. Is there another molt between this and the adult stage? They can hiss pretty well too.

Hello,

A subadult hisser is 5-6 cm (2 inches) long and 2,5 (1 inch) across. I think yours are juvenile.

Best regards,
Javier.

Yeah, Javier is right. They still have a couple of molts before adulthood. Just keep them warm and they'll be adults in no time.