#1
Could the 12" Celestion G12T-100's be used as a bass speaker in a bass cab? I have an empty 4x12 Peavy 6505 cab that I want to turn into a bass cab and bass speakers are very expensive, but I can find a 4-pack of these for $199. Just for home use, no gigging.
My gear...

Samick Les Paul (AV6VB) Guitar w/ SD Alnico2/JB
Marshall AVT150H halfstack
Schecter Stiletto Elite-4 Bass
Ampeg BA112 Bass Amp
#3
These speakers sound awesome.......for guitar. I still don't know why they are selling those speakers so cheap? Had one in my old valveking and cleared up the muddy distortion big time and had a nice clean sound to them. They do not get enough credit for how good they sound because they do not have the word "Vintage" in the title.
My Gear:

Fender Strat w/Duncans
Hand Built(by me) guitar w/Duncans
Ibanez RG321MH
Bugera V22
Peavey Vyper 30 Watt
Xotic BB Preamp
MXR Carbon Copy Delay
Digitech Hardwire Chromatic Tuner
Fender T-Bucket Acoustic
Digitech Bad Monkey
#4
Unless it says guitar and bass then a bass will tear em up. The freqs a bass hits and the amount of speaker movement in more than most guitar speakers were made to handle. Have a lok at eminance speakers they have a good selection and not any more than guitar speakers.
#5
NO BASS THROUGH GUITAR SPEAKERS, it will rip them apart.. literally.
Mesa Single Rectifier
Marshall 1960A vintage
Rg3exfm1 w/ EMG 85/81
Big Baby Taylor Acoustic
Ibanez TS808