Could I hurt either of my amps if I connect them like this: Kustom SS amp speaker out > 3 foot speaker cable> to female/female 1/4" connector > Vox AC15CC1's speaker cable(amp is off) ?
Only if the impedance is wrong. If you're going from amplifier directly to another speaker and the impedance matches, you should have nothing to worry about.
this reminded me of something, although i know the above is for ss amps.
never turn on a tube amp without a speaker load connected, you can blow your output transformer.