Using Keylight to get rid of background

Keylight requires chroma values. Your stock footage has a white background. The only option I can think of is to try and create a black and white version of the stock footage where all of the color values turn black. Yo'll have to live with the highlights. You can then use that high contrast black and white version of the confetti shot as a track matte for the original footage. It's not going to be great, but it will be the best you can do with the footage sample you provided.