How old are the wethers? Presumeably at least 4 months - in which case, I would have said, that if they were going to get full-blown horns, they'd have proper little horns now, not something you would describe as 'stubs'. So they probably will always have just stubs.
I wouldn't be worried about them damaging the Hampshire Down ewes, but do be aware that headflies love horn stubs, so you may find you need to spray their heads with Crovect, or apply a fly repellent very frequently, in the fly season.