Congrats on the purchase. That's exactly how I started on the slippery slope that is Festool, CT26 and TS55. That was AFTER I built a couple of cabinets and shelves for our walk in closet... My wife saw how much of a pain I had using a cheap Skill saw and home made straight edge.
For connecting the two tracks check out TSO's new guide rail connectors. About the same price as the Festool version and it looks like they are a lot more fool around proof. You'll want something quick and reliable when you are doing rips down the 8' length of plywood. And get their GRS-PE guide rail square while you are at it.

And the pink, hard foam insulation works great as a cutting platform. They come in 1" or 2", either works.
I took a 4'x8' sheet and broke it into two pieces that fit my MFT's and make it easier to store.
I used the foam on the floor before I had a workbench or MFT. Which, I would strongly suggest looking into the MFT next. It's excellent for clamping and cutting on. Either that, or you might want to look into a router like the OF1400 - if you don't already have a router. The Domino looks great, but with cabinets there are a ton of options for joints like rabbets and dado. And you can easily make those with the OF1400 and the tracks you have.
Like the TS55, the OF1400 was one of those game changers for me. I couldn't believe I'd gone so long without a router. And to get one that makes it so easy with the rails and accessories.
Also, check out the Woodpeckers box clamps. They work really well for helping you get square corners when assembling a cabinet.