Choosing your own number of keys only works for osu! standard beatmaps. Those songs are not made for osu!mania. Songs made specifically for mania are fixed on the key amount the mapper intended their map to be.
Here are the difficulty symbols for all the game modes:
Star shape = osu! standard
Drum shape = Taiko
Note shape = osu! mania
Fruit shape = Catch The Beat
Here are the difficulty symbols for all the game modes:
Star shape = osu! standard
Drum shape = Taiko
Note shape = osu! mania
Fruit shape = Catch The Beat