you should be able to use the mixed race list in racial weights for terror missions and have them pop up from time to time
Mixed might not be the best idea as the terror mission deployment gives you everything but the highest-ranked aliens of a race. For mixed race these are Ethereal soldiers but the two aliens below (The two classes of aliens that will show up in the mission) are Ethereal Leaders and Ethereal Commanders respectively. You may not want these early in the game.
EDIT: I've made a ruleset which changes the mixed race definition to have all terror units and makes mixed crews perform terror missions. You should get all terror units or all terror units and 1 or 2 snakemen soldiers. It's attached at the bottom of this post.
To use it, add - TERROR CUSTOM below - Xcom1Ruleset in your config file and place the ruleset file in ...\openxcom\data\rulesets
Let me know if you have any trouble!