Surely you can. Wouldn't you be loyal to your own startup if it turned into a corporation? Any way, you should be loyal to your employer while working there, but that shouldn't be mistaken for selling your soul to them.
I don't see the problem with being a job hopper except it sounds really hard to jump jobs once a year or more often considering it takes about half a year to get settled into a new job.
Personally I've set myself a 5 year limit as max time to work at one company before moving. This is part of my not-form-lifelong-habits program.
So even if I do have my dream job I force myself to leave after 5 years.
Forced perspective is good for you, you should always try to push forward and you can't do that by sitting tight in a job.
I would be skeptical about hiring someone who switched jobs 6 times the last 5 years, but I'd be even more skeptical hiring someone at 30 with less than 5 jobs on their CV as that might suggest them being lazy sods who didn't have summer jobs etc.