I have been using the latest version of the 10'4" 2wt Moonshine Epiphany for the last year and really like it. I have started carrying a second reel with a conventional fly line and dry-fly leader. For me it seems to take less time and fuss to just switch the reel rather than cutting off and storing the mono rig section and replacing it with a conventional dry fly leader. I was experimenting the other day with casting distance and I can throw over 50ft with a 5 wt line on that rod. It might be a little over-lined, so maybe a 4 wt would be a bit better. But it's surprising that it casts a 5wt line so nicely and accurately.
I have also been experimenting with a fairly heavy float indicator on a mono rig for casting to 30' or more to cover mid-stream pockets behind boulders. The long rod and light mono line can hold the intervening line off (off the water above) fast-moving chutes and get a decent indicator drift in these pockets. It's not a pristine drift, but in a big river like the Lehigh (deep and fast at many places in the gorge), it lets you get a cast and decent drift in boulder pockets much further out into the river than you can with a basic mono rig.
This is sort of meant to highlight that the mono rig opens up some different capabilities (besides just tight-lining) that a conventional fly line rig can't do.
With that said, before you buy any new gear, you can use any fly rod to try out the basics of using the mono-rig. I would start with as long and light a rod as you already have and experiment with it. The main difference is the reach you can get with a longer rod. It's surprising how much more water you can reach with a rod that's just 1.5' or 2' longer than your normal 9 footer. Yes, a 10' rod is an okay starting point but if you're buying one just for tight lining, I would say go to 10'6" or 11'. My first tightline rod was a 10' rod but I only used it for a year or so before I went to an 11' rod. The 10'4" I mentioned earlier is a bit more versatile in that it also works great with a conventional fly line and dry fly. I find that it's good enough for tight-lining most of the time. If I am pretty certain that I will only be tight-lining, I will take my 11' rod. I'm not a fan of carrying two rods.