Re: v30 and g12h30 w/ 100w amp
yes, double the lowest wattage speaker's power rating, in this case.
I wouldn't do it. Amps are usually rated at some %THD (total harmonic distortion) which in practice means that the amp will be putting out something like a 100W when the power amp begins to distort. On something like a marshall, this often means somewhere beween 3 and 5 on the master. So unless you want to run the amp on just 3 all the time, I wouldn't run those two speakers.
You could try an eminence wizard, which most people claim is a good substitute. It handles 75W.
But even then, many amps are capable of producing more than their rated wattage when dimed. if you dime the head, there's still a chance of burning out the V30... but for the life of me I can't name too many people who run their 100W heads full tilt all the time, so you'd probably be OK.