Any sorcerer can make Shikigami if they spend time on it, like how Yuta can make a bunch of mini-Rikas whenever he wants. In fact, JJK0 suggests actually that Rika was never originally part of his CT, but became a part of his CT when he cursed Rika's spirit, binding them together Most sorcerers don't use them in combat though because they're hard to control and reduce your ability to fight, hence why that one dude Geto fought figured that he'd hang back and let his spirits fight, since shikigami users tend to let their minion fight for them. Geto never had that limitation because Cursed Spirit Manipulation doesn't require him to directly control the spirits at all.