Is it better than buying Head original Heel-Toe replacement set for 14usd?

Protection is only one benefit of the Yaktrax product. If you only want protection, you can get Cat Tracks. What I love about the Yaktrax is the curved toe and heel which make walking in ski boots much easier. They also provide a bit of cushioning when you step. For me, they are absolutely worth the money and worth carrying around on the mountain. Also consider that if you keep your boots long enough you may not be able to obtain replacement toe and heel plates.

Do these work for grip walk boots?

It's hard to say for sure without trying it. They should still fit the boot but these are designed to let the foot roll. Since the grip walk soles are also designed to let the foot roll the combination of the two may be too much.

I have a pair of Solomon Mondo size 26.5 boot with a 306 boot length (i.e. right between a small and a medium size). Is it better to size up or down?

I had to go with a small to fit my 26.5 with a 308mm sole. They were a really tight fit but they ended up stretching out with some use. The mediums would just fall off. I ended up losing them after a few months when they fell off my boots while on a chair lift (uploading without skis). Since then I've just started purchasing less expensive options

My ski boot length is 294mm size 25/25.5 am I a small or would a medium fit well also?

I would suggest the small. My boots are 27.5/316mm, and the mediums is just small enough to fit. I think any boot shorter than about 305mm would need a small.