Getting tiles in plane -- What if use a *too large* notch trowel?

Question

Is there a consequence to using too large a notch trowel, aside from wasted thinset? Asking for 2 reasons:

1. I'm installing 16x24 tiles on a floor, with 5x5 "accent strip." Both tiles ~10mm thick. But if I use standard trowel sizes (1/2" for big boys, 1/4" for small), the thinset-thickness-difference could cause lippage. So is it okay to use the 1/2" for both tiles? High traffic area in a shared bath, plenty of point loading on those 5x5s.

2. I'm tiling a shower with Kerdi Board. Before I installed the Band, my walls were dead plumb & flat -- but not now, thanks to buildup at overlaps (e.g. around niches). IMHO, I installed it perfectly, using All-Set, with no excess thinset. There is clearly lippage if I dry fit a few 6x12 wall tiles. I was gonna use a 1/4" notch here... what if I use a 1/4" x 3/8" notch instead? Or maybe a leveling system? In lieu of floating the whole thing... These are not the flattest 6x12s I've ever seen... Thanks!

Answer 1

No one I've ever worked with changes trowel size mid-floor. Use the larger notch throughout. If you find that the smaller tiles are sinking in further, back butter them with the 1/4" trowel so there's more mortar under them. In all cases be sure that edges are well supported.

Regarding #2, use whichever trowel gets you good results. Tiling is an art, not a science. As long as every tile is well bonded and supported, it's fine.