I haven't had any trouble getting my 828 interceptor to intonate properly at the 12th fret, but I've found it's a bit iffy at the 24th fret on some strings. Also, the action on mine came very high - you might have to shim the neck or perform some other mods if you want lower action without...
I just got a 28" 8-string after never having played a seven, and while I'm still messing up sometimes, the transition has been pretty smooth. I think I've almost got most of the muscle memory stuff down, and it's been four days. I'd go for an 8 if that's what you want to play.
There are a bunch of threads about strings already.. they might be on page 2 right now. I got a pack of D'Addario EXL110-7 (seven strings, 10 to 59) and a single NW072 (also D'Addario). I ordered from JustStrings.com - Strings for guitar, bass, banjo, mandolin, fiddle and other musical instruments.
I don't think taking your existing riffs down is that great of an idea. If they already sound good, why bother? Instead, see if you can spice some of them up with the 8th string - add a note an octave below to accentuate a chord, that sort of thing. Play around with tuning the top string (or...
Just a heads up.. bunch more just got put up, some sevens as well. The 828 Interceptor in white is especially gorgeous. Jeeeeeeeesus. Rondo Music Electric Guitars
By "even fret" do you mean the 12th fret? If so, there are calculators online for that.. just look for scale length / fret spacing calculators. I think most of this is going to depend on your tastes, by the way - maybe tell us what kind of sound you're going for?
I grabbed one of the last three Interceptors two days ago. Only my second guitar, and my first with more than six strings. Can't wait for it to arrive! djent djent etc