Can I use both in the same project? Or do they cause "network interference" with one another?
You can use simultaneously any Ajax mechanism in GWT, they are not exclusive, so you can select the most suitable one for each necessity in your product.
For instance, I have an application which uses RF for entities and business requests, gwt-atmosphere (which uses RPC) for comet comunications, and gwtquery-Ajax (which uses RequestBuilder) for consuming 3party json services.