How can I create Motion titles with fixed-length transitions?
You can use Build-In and Build-Out markers. If you right click (or control-click) on the top of the timeline, you'll get a pop-up menu with an option to add a marker. When you add it, you'll get a green timeline marker. Double-click it to edit it and choose whether it's a mandatory or optional build-in marker. Everything before that should play at the same speed in FCPX as it does in Motion. The area between the build-in and build-out markers will stretch to fit, and everything after the build-out marker will play as it does in Motion.