Yes. A mistake which can make this not true is if you configure one client and then copy over the configuration file from that install to a client on another system or in another folder (for a different CPU core). Each client needs to be configured on its own so it will get a unique numeric ID and thus will also get unique work units. Of course, with proper setup procedures (just don't copy to create identical F@H folders in multiple places), you can use the same username on as many systems as you'd like.
Yes. Some work units have bonus credit associated with them (such as units for the GPU and SMP clients), so the credit/time scaling isn't linear for all work units. However, in general, you are correct.