Can I apply Clustering algorithms to the result of Manifold Visualization Methods?
You can do anything you want in the low dimensional space, and can try to validate as well. By clustering the above, you are in effect assigning features/tags to your data points in higher dimensions. Remember, tSNE tries to preserve distances, so that points in high dimensions will remain close to each other in low dimensions. 

With that in mind, don't forget that no two instances of tSNE will be the same, which means that your clustering centers will be different each time you run tSNE.