Welcome to FOG
If you already have a rail you can set the rail on the cut line you desire and cut that guitar, even doing so right it sits now. The rail doesn't need to be clamped as long as you are careful, but clamps are always a good thing. So I would recommend some of the Festool quick clamps.
Although some folks protect the top of their MFT's the other option is to just cut into it. In other words set the saw blade to cut 1mm or 2mm deeper than what you are cutting into. Others put something down that is used as a sacrificial surface.
Another option instead of the rail setup attached to the MFT as a bridge, is to use "Rail dogs". These are either slide into the grove of the rail, or you bottom screw them into the MFT and rest the rail up against the side. Dogs are a handy item to own too (lots of options), as are the Festool "Clamping Essentials".
Make sure to run the saw down the rail and cut the strip to your saw.
Some different saw blades are going to be in order too. Tenyeru, and Oslun have cost effective blades for the TS55. You are going to want a ripping blade for cutting that wood along the grain. The blade that comes with the saw will work though.
Check the classifieds on this forum for someone selling their MFT "extra parts" as many people do not want to use the fence nor the cutting rail "elevator". Many people use other options for their fences even if they have the factory fence.