Most of the ride is quite flat, excepting the bridges, unless you're doing the full century. In that case be prepared for some lovely hills in the Bronx and Northern Manhattan. Fun to have those grades on the last 15 miles!
I ride with 23s on my road bikes all over the city, but they're Nashbar's kevlar tires. 25s should be fine, but you'll be ok with 23s, too.
Last time I did the century there was a guy on a unicycle keeping up with us the whole time. I saw him at every single rest stop. If he can finish it on his "single-speed," there's no reason anyone else can't, either. I've had a couple friends do it on single-speeds, and they were fine, albeit zonked. Just remember about those hills at the end!
Since you're doing the 55, I'd say ride whatever tires and gearing is most comfortable. Finish up those miles on the bike you like the most, then on subsequent tours and races you can challenge yourself by riding single-speeds and what have you.