Your questions are not ignorant but should have been ask before you bought that thing.
Almost all the barrel nuts, if not then surely then mount that goes around the nut, is specific to the rail.
I prefer Knights Armament for FF rail systems and accessories; they make some of the best parts on the market and they supply the military --I used a LOT of their stuff when in service and still use it today. They've never let me down and I like the clip in place rail covers that make it feel like a nice FF tube in places the rail isn't used. They make a few different ones, the regular FF tube with rails down it (the M5 and carbine FF?) in a few different lengths, and then they make a few different types of the URX rail, my current favorite for carbines in mid length. There's one with a built in front flip sight, and the rear micro KAC flip sights are some of the best you can buy and deals can be had on Ebay for those. This rail requires a special wrench to install (or have someone do it) but it has a lot of features built in including sockets for QD sling mounts. They make 'em with keymod, regular rails, removable rails, removable panels, etc.
You can find the rifle length FF KAC rail system, the M5? It's the "older" one but it's still in wide use --it's just a different one. Mind you it can have some minor flex at the end if you use a tight leather sling for instance. Otherwise it's great with no problems. I use a couple of them, one on my tack driver.
JP Ent. makes a nice one too, V-Tac? if you want a good tube with slots where you can add rails only where needed. Some of the URX systems are like this too. I'm not sure how well it mounts, as I didn't get one, I got the older KAC instead. PRI makes a nice FF carbon fiber tube that's pretty solid. It's not all that light, at least not the short carbine one I got, but it's very heavy duty and well made looking. The triangular grip feels really nice too and these only have rails where you want them and you can get top rails that connect the upper to the top of the tube to strengthen it more (though this doesn't appear to be necessary with the short carbine one I have). Mine went on VERY tight and I expect no movement. All of the above work with regular mil-spec uppers and even many aftermarket, billet or other uppers like the VLTOR MUR-1.
Daniel Defense got the new contract for the new SPR rifle I think (the fourth one?). It uses a rail that bolts directly onto a special upper receiver designed to accommodate the rail. This method of attachment is likely going to be the most stable and strongest of the bunch, save a one piece upper/forearm maybe (monolithic). Both of these methods will likely cost more than a FF rail system on a mil-spec upper, but I think Rainier Arms has some monolithic uppers for about $600 or so. Mega? And there are other bolt on rails to uppers, I just can't think of them off hand and I don't know how solid they are. The DD is likely gtg though as it's a military part now.
I personally am not a fan of any FF system that just uses a clamp (Olympic Arms) or worse, set screws to hold it down. I can't believe POF does this, who I thought was a higher end manufacturer. I must be missing something... It's gotta thread on first and then use set screws in the right locations to keep it from moving, right?
But I do know that even good rails systems can flex under hard use/abuse. The FF KAC rails, the older ones, and many others too, they can flex at the end and the longer they are the worse it gets (making use of clip on's and lasers, etc., less accurate when held tight with a shooting sling for instance). I don't notice the same flex in the URX systems though, and you shouldn't have any in the one piece or bolt on types.
Interestingly, I don't have any flex in my non-FF KAC rails. While most of the weight is kept to the rear with these, they do affix to the front behind the sight assy. and distribute weight and force along the barrel. Yet when compensated for and good dope and data taken, and accessories left in place once mounted, they tend to be nearly as accurate within a couple MOA, depending. But FF rails will almost always yield tighter groups regardless.
It's certainly worth doing this once and doing it right. Expect a good new rail to set you back $300+ unless you can find one used that's exactly what you're looking for. Look on here and look for sales online. Operations Parts has lots of hard to find KAC stuff if you're interested, and Rainier Arms is always a good place to shop for new and different stuff, all AR dedicated. Bravo Company sometimes has good stuff in stock too.
Good luck. And if you want keymod, look at the KAC URX (there's a keymod version but like with all KAC stuff, all the accessories are extra, so choose wisely) and there are others including monolithics and bolt ons, but I can't speak for how well they work.
