My wedding receptions are very much like club environments, very loud dance floor music. Up until now I have been using my HMC150 with a Rode NTG1 on one channel and internal on the other (forgetting for now a line feed from the DJ).

I know shotguns on cameras are not the best way to capture sound but for run & gun it can be the only way. IMO the internal mic on the HMC sounds a bit clearer than the NTG1. Note: the HMC doesnt have an 'auto levels' function.

Now I am going to use my spanking new C100 I'd like any opinions of the performance on the internal mic in this type of loud music environment. Does it hold up well? Can I ditch the extra bulk and weight of the Rode and just stay with the internal?

I think for that kind of environment you'll be just fine. Internal mics on C100 (and the audio set-up in general) is all very good. I have noticed some tendency to accentuate bass with the internal mics though (not a scientific test, just some random testing when filming one of my kids at a school play).

Also, just so you know, if needed, you can activate the handle's inbuilt mics Attenuator which gives a 12dB cut - might be useful for your needs (see page 75 of the Manual). If not, you'll get a 20dB cut with any external XLR mic you attach with this attenuation selected.

I found that the internal mics are far more susceptible to handling noise than using a shotgun. They also picked up the IS noise from my 24-105 which the shotgun did not.

shotguns suck for receptions. Once the toasts are over, off it comes, and all internal. Make sure you have a line feed from the band or DJ as well.

Fair point about the IS noise with some lenses. Luckily my Canon 17-55mm F2.8 has a remarkably quiet IS system so I'd forgotten about that.

My Canon 70-200mm F4 IS is very noisy - for sure the internal mics pick that one up!

shotguns suck for receptions.
Too directional?

it's your first go with this new rig, use the internal and ntg-1 and decide for yourself which works best for future shoots