The 151 is about 8.5cm wider than the combo, so the latter should sit nicely atop the former. Should look something like this?
Yes they won't match exactly, but does it really matter if it sounds good?
I've mixed driver sizes many times over the years and never thought it sounded bad. More than anything, I think the 15 will just make your sound "bigger" and more rounded.
If I was running said setup I'd dial the piezo out of one of the cabs entirely, probably the combo. I might also think about angling the combo up so that it becomes more like a monitor. This might also detract visually from the different speaker sizes