Amp and Speaker Wattage
 

I own a 100 watt Kustom KGFX head and I have recently come into possession of a 212 cab with 75 watt speakers. What's going to happen if I let these crazy kids hook up? Blowing my speakers or what? I was thinking if I keep the volume down, it would be okay. I, however, play with a really loud drummer and have to let my volume set at around 8 to be heard at practice.

You need to consider more than just wattage, the main factor is impedance (measured in Ohms - looks like an upside down U). Normally it's going to be 2,4, or 8. Check out these links for more info

AV and Automation Industry eMagazine - Speaker Impedance, Your Amplifier And You. | HomeToys

Speaker Impedance Explained - Ohms

The cabinet is a 4 ohm cabinet and the head is meant to be used with 4 ohm cabinets, so I don't know if that is going to be a problem. I'll read through those articles though. So If I max out my volume I won't blow the speakers based on the wattage alone? Like I said, 75 watt celestions and a 100 watt head on 10.

If you put 100 watts into 75Watt speakers you will blow them.

If you attenuate to 75 Watts or don't play at full volume you might get away with it but guitar signals have very high peak transients so its not guaranteed.