I have a 90 degree adapter on my 7' long hose, and I keep meaning to take it off. It's just a remnant from when I was using a 40" hose routed down my chest for my primary. It does curve the hose directly back around my head, which maximizes the effective hose length, but that doesn't seem necessary since I have so much extra length anyway. Mostly it makes it more difficult to clip off my primary when it's not in use because the angle sort of puts the reg in the way of getting a good grip on the bolt snap.
You will want a 70 degree adapter on your necklaced octo, though. You can buy a premade necklace or make one yourself out of shock cord (3mm bungee cable). Instructions can be found on youtube and it's an easy way to save a few bucks while learning some handy knots.
Plenty of websites (scubatoys, diverightinscuba, divegearexpress etc) can sell you hoses in any length you like, but depending on your experience level you might want to have a qualified tech switch them around for you. If you over-tighten the connection points you can damage your regs.
I don't know much about the ABS octo, but my philosophy is to ensure that my primary and backup second stages are both fit for heavy use as primaries, so I gravitate away from anything marketed as a backup. Just a personal preference.