Okay, so here is the update so far.
1. After signing up with Yahoo! Calendar Beta, that seems to have helped, with a few minor quirks (see 2 and 3).
2. Every few days, my Yahoo! Contacts, or Calendar, or Email would require me to type in my password again. I think this might be due to Yahoo!'s tendency to expire the cookie password. Re-typing in the password fixes the problem for a while.
3. Then maybe every 2-4 weeks (didn't really keep count), Yahoo! password from 2 above wouldn't take. Even if I type in the correct password, it would keep telling me I have the wrong password. To fix this problem, I just log into Yahoo! from my computer, sign-out, then re-sign back in. Then when I type my Yahoo! password on the Pre, it works, just like that. Go figure.
So for now, I have Yahoo! Contacts, Calendar, and Email co-habitating on my Pre. I haven't tried Yahoo! IM yet so don't know how this changes things. I'll get to it eventually and will post results.