Using Suits as an example as you did. Harvey has great definition and size and can fill out a regular fit suit. Kids going to prom don't have that as they have't grown into their bodies yet and as such don't match the proportions of the people the cuts of the suits were designed for. Regular and slim fit suits are for two different builds, basing it on what an incrediblely well dress lawyer wears isn't a good idea as it is a very formal , very expensive attire.
As you are 6ft and only 70-75kg I would 100% go with the slim-fit suit and shirts, otherwise they will look to boxy on you. Harvey has tailored suits don't just it by that.
FYI I'm 6ft1 and 93kg lean, and I still wear slim fit shirts and suits just because I don't like the stiff and boxy regular cuts. As the jamesthehhustler who advised against slim fit because apparently you can't walk comfortably in them, he is wrong. The right cut and the right size come into it, slim cut is for slim people and if you're 6ft and 70-75kg with a young persons proportions then 100% for the the slimfit, and pleaseeeeee don't get a hat.