How is the waterpik for dental health? Should I use peroxide in it?

Excellent. It is an excellent addition to brushing and flossing. I would not recommend using peroxide in it. Mouthwash is popular, but remember to run some clean water through it after using some other liquid. This will help keep it working properly and help prevent it "gunking up" or getting clogged. Good Luck.
Good. and Don't. The water pik can be part of the regimen to maintain good dental health. But it is not necessary to accomplish this, and if other techniques are not followed, it is not enough (see prior answers about good basic dental care). As for peroxide, would not use straight peroxide on any part of skin or mucous membrane or in mouth: too astringent. Perhaps diluted, or combined with baking soda, may be ok.
Terrific. Helps clean between teeth, remove food debris, disrupt bacterial accumulations, and tone up gum tissues. Best to use with just water alone. Not a replacement for brushing, flossing, regular dental check ups.
Yes, but not alone. Imagine trying to wash your car with a high pressure hose without touching it. Plaque is adhesive enough withstand a blast of liquid alone. In support of waterpiks, they are excellent at removing larger particles from tight spaces like braces bridges and implants. Its best to floss and brush also. Hydrogen peroxide delivered with a water pik can help keep your gums healthier.
The #1 recommended. by dental professionals. Provides a unique combination of water pressure and pulsation to massage and stimulate gums, removes debris and bacteria and improves oral and general health.