Whether you use a testimonial page or not, it's a good lesson in setting up a child page under the parent "About" page. Building a "My Clients" page as a parent would make it a tab on our site. We're going to be hard-pressed to organize with the space we have for tabs. Also, a lot of tabs only confuse and frustrate your visitor. Giving them logical tabs with sub groups will work better.
We'll do a quick setup of a page titled "My Clients." Note that we select the About page as the parent near the bottom of the screen. The next screen shot will show you how this theme displays the results...or doesn't!