Unfortunatly, I don’t think there is a way to remove the Guardians and Ultralisks with just the base editor. Afaik, these units are written into the A.I. script that the Computer uses to function and the only way to change it is by using mods or tools which would then alter how the game is runned(could use another person to explain this more accurately).
Edit: Found a link to a thread that might help(I haven’t tried this myself yet). Hope that this is what you’re looking for.