i've also been one of the people asking for something like this for awhile. even though your actual perk level might be higher, you can only use the maximum level as set for that difficutly level.
i suggest:
Beginner - max level2
Normal - max level4
Hard - max level6
Suicidal - no max needed
on the other side of the issue i think it would be fair to set a minimum as well. in this case, you'd have to use a perk that was at the requirement the whole time, so actually you'd not even be able to join the server unless one of your perk levels met the requirement.
Beginner - no minimum needed
Normal - min level1
Hard - min level3
Suicidal - min level4
i think those requirements are pretty fair. the main object is to stop the mixing of "extremes". no more level6s griefing in normal, no level2s griefing in suicidal. open to suggestions in regards to the numbers if they are too high or too low.
i tend to favor standardizing in this case, simply because i am convinced that not all server admins will use the most ethical judgement on their servers. just like there is a whitelist because TW does not believe that all mappers will make proper maps and will try to slip in leveling maps. you can see an obvious example of this when you saw suicidal servers running the poundamonium mutator on KF-ConcertHall or KF-Defenceb2
remember standardization doesn't completely restrict all freedom, it just slightly limits freedom to help keep the most fair and balanced playing field for all players.
i agree about level6s, that's why i suggested the max for hard would be level6 so that allows level6's two difficulties to play on but keeps them from abusing difficulty levels far below where they should be. oh and for the other thing you mentioned, i'm happy to say i don't think many suicidal servers, let alone ANY server will be
running that map anymore