I'm sure I'll get smacked for this, but it's the truth...
Adding rails to your existing handguards is a sloppy idea. Your handguards weren't designed for it. They're too thin.
If all you want is a vertical foregrip, and you're not in the Navy SEALS or anything like that, there's not a single thing wrong with a set of FOBUS M33 handguards.
Don't pay full price for them though... If you look around, you can find them for as little as $65 brand new, or for $50 or less used on the Equipment Exchange.
They are noticeable thicker than factory handguards, and they lock up tighter than factory handguards. They don't wiggle and have slop (at least the two sets I've owned didn't). Despite what you'll be told soon, they're not "crap" either.
You shouldn't use optics on them at all, and they're only good for lights and foregrips. A TangoDown foregrip may not fit, but your TDI absolutely will.
Yankee Hill handguards ARE BETTER than the Fobus. But they're more expensive and heavier. Daniel Defense is better still, but even MORE expensive (and lighter). In fact, they couldn't compare favorably to any aluminum railed forend, but hell, they're not aluminum! What they are is a very nicely constructed set of HANDGUARDS. They're fatter than carbines, slimmer than M4, and have rails that are lower in profile than bolting some on to your existing HG.
The question is, how much do YOU need? I have owned 3 DD forends, and they're great. I've also tried YHM, and PRI is next. BUT I have this one particular carbine, a 16" Bushmaster superlight, that's designated for the wife and kids to shoot. It has a pair of Fobus M33's on it (which weigh the same as the DD 7.0 without any rails) and it does the job it's intended to do. I have a TDI short grip on it, and I can hold the rifle by the grip and the handguards don't move. They don't flex, they don't bend etc. I'm sure I could make them flex, but I'd have to be intentionally trying to break something at that point. No amount of range use is going to break the Fobus or the TDI.
It all depends on what you NEED and how much you can AFFORD.
But to answer your FIRST question... Bushmaster sells a bolt-on rails (made by Yankee Hill of course) that attach to your handguards and are inclined to match the taper of the handguards and make your rails parallel to the barrel. I'd still rather have the Fobus instead.