The only reason the side kage don’t receive things are because they don’t have to be tested for. They receive a hat, nickname, and a skin like the other kage, but until they require a combat test, they aren’t going to receive any special perks.
In the future, we plan to make side kage require a combat test like the rest of the characters, which would give them jutsu perks, but there is a list of things that are more important. Anyone who got the character when it had no requirements would have to test to keep the character.
At this point, these side kage would just be official kage.