Having a testimonials page on your website is a great idea. It shows the clients going to your website that you are a good provider. I would think you could just place "Review from Google+" or "Review from Yelp" at the bottom of a comment.
Here is our testimonial page: http://www.essentialpest.com/testimonials/
We don't place reviews from other websites. The testimonials we have posted came from emails, letters, etc. - However, I would think that if you linked the word "Google+" from "Review from Google+" with a link to that review, it would just be proof that review does exist.
This may be useful to you. http://moz.com/blog/getting-reviews-the-right-way-for-local-businesses
Good luck!
