You can do a century on anything that rolls and is human powered! on the last one I did, Cruisin' the Conejo. One guy used a unicycle! I think he may only done the metric. You wil probably find people on hybirds on the ride. I did my first century on a hybird.
I would second the advice of looking at the start/finish time. I usually like to start as soon as I can, just in case. I think most rides have a 10-12 hour time limit.