Yes they work fine for 12x12 clay. Sooteater is a good homeowner grade one.I'm sure your much more knowledgeable in this area. And for this reason felt was likely wise stop this practice having heard about some rotary brush was going to look into that supposed works in 12X12 masonry flues, is that true, if so any you'd recommend?
But 12x12s are drastically oversized for just about any stove. You would benifit greatly from properly sized liners.