Also, Slimers aren't necessarily better than game play acquired sidekicks. I leveled a Slimer and when I was done, it still had lower total skill points than the little chicken I leveled just before that. But for the record, you can always reset a sidekick's skill points to reflect your current level by teaching it a skill from another sidekick.
Now, just an fyi. A sidekick has to have already learned a skill before it can teach that skill to another sidekick. Teaching a sidekick a new skill returns the sidekick back to level 1 and makes the teacher sidekick disappear. Also, teaching a sidekick a skill costs 15 donuts. And lastly, sidekicks cannot learn skills they already currently know.