First off, Focus SRS components (air bags, seat belt buckles, etc.) are model year specific. As you probably know, the SVT seats have side air bags, as well as a seat position sensor on the driver side seat track. If your SE seats do not have side air bags, or a seat position sensor, the main thing to remember is to replace the SVT seat belt buckles with those from your SE seats. If your SE does have side air bags, you will need to swap the SE air bags into the SVT seats, to avoid an air bag light, which indicates that the entire SRS is inoperative. I certainly wouldn't advocate, or advise driving with a disabled SRS.
However, if the only issue is the seat belt buckle, a switch to SVT seats will give you a huge improvement in comfort. I made such a swap into my '05 ST, and never regretted it for a moment, once I got the SRS issues resolved.
The SVT seats each have a "master" plug, that is, a plastic shell, which each of the four or five under seat wires plugs into. Each of those under seat wires has it's own unique plug, and their wires come from air bags, seat belt buckle, seat heaters, etc. Those four or five individual plugs can all be removed from the "master" plug, and plugged into the appropriate, corresponding individual plug from the SE's master harness, which come out from under the carpeting, under each seat.