Copied over from Campaign Discussion:
I completed the missions in the following order:
Char-Zerus-Kaldir-Space … currently going through Skygeirr.
I completed all of the evolution missions as they became available (Zergling, Baneling, Roach, Swarm Host).
Prior to Skygeirr, I had not completed, nor had the option to complete the hydralisk evolution mission.
The Hydralisk evolution mission finally appeared after completing Skygeirr 1: Infested.
I expected to have access to the hydralisk evolution mission right after getting the hydralisk in Kaldir: Shoot the messenger, since I had already completed Zerus: Supreme, which triggers the hydralisk evolution mission.