I am not sure it can work like that - levelled readers should always be supplemented and also children should be reading some books at an easier level and some at a level they need some help with so they can advance - probably my advice would be to get a few (at least 3 of each level) and then continue to use the library readers even if they are all over the place. Competency comes with wide practice and also often re-reading certain books to gain fluency.
I agree! We are all over the place too. We use I See Sam books(we are on set 3 and we read one or two a day) Mercer Meyer books, The Reading Lesson, Bob Books, and any other books we can get our hands on. If they are at my DS level or lower he reads them to me, if they are slightly above his level we take turns reading pages and I help him figure it out.