You are correct brian. While we have no evidence that gravitons exist, both General relativity, and particle physics indicate that they do not travel faster than light, In fact, because they are a massless particle they must travel at the speed of light, the same as photons. This means that they cannot travel faster than light, nor slower than light, but at exactly the speed of light.

Your last question is indeed interesting, and one that should hopefully be answered if and when we ever get a working theory of quantum gravity. As best I understand, gravitons are the quantised vibration of gravitational fields, hence they are coupled to objects that have mass (just like photons are coupled to particles with electric charge), and so interactions between particles with mass would be through the exchange of gravitons. Therefore, gravitons may have a degree of coupling with mass giving fields.

Jonny