Wedgek11°-23° and 24°-43° Combined, X-Large Angle Guides for Sharpening on Stone, Model LR5 by AngleGuide

Q: How do these hold up? What kind of plastic is used?

It is a hard plastic. I personally found them useless. But it could just be me. I found it difficult to continue the angle to the end of the knife. I prefer just estimating the angle. My knives are sharp just using the stones and estimating the angle. Used 1000 and 6000 stone. going to buy an 8000 for an even sharper hone.

Q: How do i find the angle of my knife? If i dont know it can i just sharpen it to a new degree?

As to your question 1, it's geometry. As to your question 2, you can change the angle of the cutting edge to any degree you wish. (90 degrees is the limit, but then you would have a hammer, not a knife.) Different cutting needs typically use different angles. As an example, chef knifes usually sharpened to 15-20 degrees. Outdoor knives generally sharpened to about 40 degrees. It's a balance between fine cutting and keeping an edge sharp in the field. Internet has good information on this subject.

Q: I'm probably going to buy these to help with sharpening but are there 12.5 zwilling ja recommended that for me?

I hope you weren't influenced by hatuletoh's answer. No 0.5° angle guide is, or guides are, included—integer angles only. That's not bad. 12.5°/side is to form a 25° inclusive edge angle. With this set you can theoretically achieve 24° or 26°, but how close you come depends on how steadily you can maintain 12° or 13° the length of your stroke. Realistically, either is not only indistinguishable from 25° but likely to actually be 25° along some part of the edge.

Q: I was told these guides scratch knives is that true when using them is that true?

It depends if you keep wedges clean or not. If you wipe off the abrasive dust, they wouldn't scratch. Also, some people protect polished knives with blue painters tape.