I don't really have any strict guidelines on power-level sorts of stuff besides the usual "try to keep things fair and balanced." A strong Servant with a famous legend is rightfully so, but thanks to the arbitration and game-like rules of the summoning system anyone is vulnerable somehow and nominally any class can win the war... just not necessarily in a straight fight. There's more ways to win a grail war than duking it out. My general policy, as unsatisfying as this answer may be, is always to go with your heart on things. You know if you're committing something fun and interesting to play with/against to the thread, and you can generally feel when you're putting in something that's just written to be invincible. If you want to feel extra authentic about balancing your Servant I recommend taking a gander at some F/GO parameters for similar legends or better yet looking at the Container Parameters table on the Servant page of the wiki and working up from there.

The funny thing about Fate is the disparity between what is stated in some materials and what actually happens in the stories. When it comes to power-levels regarding Masters I think the anime adaptations are king in this regard. The original VN and especially Zero tend to be a little overzealous with just how much faster and stronger Servants are compared to the 'average human' baseline which becomes meaningless and confusing when the average Magus shatters that baseline as well. It's an element of the series that has been continually toned down (not that it was ever strong when a magically roided ethics teacher punched out King Arthur and Rin took lives off of Heracles) to, I would argue, good effect. Magi are notably weaker than their Servant counterparts, comes with the territory, but it's totally okay for their abilities to contribute meaningfully to the Grail War and combat between teams.