Running a mixed group of monsters is not hard at all, and should never make one feel that their head is going to explode. Simple monster management is all it takes. I’ve been doing monster management since the early 1980s, and it really hasn’t changed. Jot down important bits of info on a notecard or piece of paper. Highlight special abilities and, if needed, flag the pertinent pages in whatever rulebook is needed. Review your notes before the game begins and quickly scan them just before the encounter. Sometimes, if the creature has some particularly special things to do, I will make a note of what their first few rounds of actions will likely be. Keep encounters orderly should be a piece of cake.