I believe a number of people run their M60s set to "large" with the sub just helping out with the lowest notes.
The only risk of damage I can think of is if you play with the volume cranked up there will be more LF energy going through the receiver so a greater chance of driving the amp into clipping and hurting your tweeters. This would be pretty easy to hear, though...
Don't think you're going to be able to hurt the woofers, unless you have a MUCH bigger amp than we think
Now that I think about it, your logic (running center/surround crossover at 100 so not wanting to run mains in Small) is probably why other people are running their 60s in Large.
