In the osu! client the only keycounts that currently allow special style (+1 or +2 keycounts) are 8K, 9K, 12K, and 16K, while you can feasibly tick the Use special style checkbox on any keycount 
and make a map with that in mind. Allowing the player to set custom keybinds for such maps should be allowed. If its in the game, you should be able to play it. For keys 3-18 mapping a map with those keycounts using the special style is possible and practical, but if there isn't any distinction between normal style and special style for certain keycounts the players and fellow mappers/modders may not realize the intended way to play the map. Also if a player does downloaded a 2-6K+1 or other special keycount map and they want to play it as intended, they shouldn't have to change their default keyboard layout every time they switch for a normal style to special style map. Pretty much, allow special style keyboard layouts for 3K, 4K, 5K, 6K, 7K, 10K, 14K, and 18K.