If you want the best quality, don't use a SCQT, export directly from the FCP timeline through Compressor to your SD master. And use ProRes works beautifully in SD, just make sure you use ProRes HQ.
Some people don't like this method as it locks up your FCP while exporting but for true quality, this is the only way to go. When you export from FCP through Compressor, it goes to the source media and ignores any renders. This way the color correction you did, the titles you made, the compositing, etc is not limited to your sequence codec or frame size.
Here's my suggestion for you, setting wise. Export from FCP to Compressor, take a SD preset that matches what you want your master to be, i.e. DV Anamorphic 24p, etc and simple switch the codec from DV to ProRes HQ. Then go in and turn on frame controls, no need to tweak, just turn it. Then put audio as pass through since why re-compress you're audio if you don't have to?
It's really important to do the resizing step separately from your encoding step, to web or DVD. Low-bit rate codecs like MPEG-2 and H.264 have enough trouble getting rid of information while still looking good, when you through re-sizing in there as well? It all goes to hell with aliasing and pixelation. Plus, once you get your SD master, you can use your cluster to crunch that down to MPEG-2 in no time...