It's complicated, we can "surmise" that everything in the tower have to do with Shinsoo: "Shinsoo is the world itself". However there're "powers" or abilities that seem don't require to form a Baang (Varagarv, Elaine).
Concerning Yihwa's "flame control" it's called "Hwayeomsa", and it's an ability to transform her shinsoo into fire. She uses it mostly with only one baang with the highest Soo her capacities allow her. It's a bit like throwing a punch augmented with shinsoo( Yuri ? :p), but Yihwa can shoot it. So her way of using it isn't contradictory with her position, as in low floors, it's concerning number of Baangs wave controlers try to show there differences (except Laure who's the most Shinsoo efficient, but this trait goes very well with his lazy nature).
Also, there are shinsoo control that are limited only by the talent/time spent on those techniques to learn. Others that are limited by being born in a certain family, or even being an exceptional individual in said family.
Animas are supposed to feed their "pet" special shinsoo (hence it's understandable why they don't need to form a baang).
How do spells work doesn't make much sense to me, you make contracts with guardian to use shinsoo and other abilities, yet you can make other contracts for spell, and even with "demons" rather than guardians, it seems. Does it remove the necessity of forming a baang ? Good question.