are lisk delegates like supernodes?
Nope , lets called them the forging nodes.
fixed income if you are a delegate?
Maybe because sometimes they missed their block.
how much do you need to become one?
To became a delegate you must have a lisk node running ,a good proposal , 25 LISK for the fee to turn on the forge features and especially the vote you will get by the community.
By the way you can earned Lisk by voting some top delegates, they're some delegates have a greater portion of their forge lisk that they distribute to theirs respective voters check this
list for more info about the percentage that they sharing.