Hello exotictropicals-
Well, I don't live in Hilo, but have visited Hilo. I'm actually on the windward side of Oahu. About your plumeria inquiry, I don't beleive you will have any problems with growing plumeria here on any of the islands. I would guesstimate that 3 of 5 homes with yards have at least one plumeria tree in them. Hilo has a lot of rain. More probably than any other town in the state. However I have seen many plumeria in Hilo and they flourish well in the area. I think you will just need to take into consideration the there will be a lot of rain. Orchids would be a great consideration as well as gingers, ferns, anthiriums and even ti.

Here is a link that might be useful: Here's a site that might give you a little info on gardening in Hilo.

Plumerias do fine! They grow huge, actually. (Well, for a plumeria, anyway. Up to about fifteen feet.) Hilo has lots of sun, I haven't a clue how they decided there is only 90 sunny days in Hilo. Maybe only ninety days when no rain falls?

Here's some URLs for web cams in Hilo, you can keep track of the sunny days yourself:
www.tsunami.org/hilobaycam.htm
www.uhh.hawaii.edu/images/camera
http://hvo.wr.usgs.gov/cam/index.htm
The third one is of the Pu'u O'o vent in Puna, it shows best at night when the lava glows.

Most of Hilo has very rocky soil, but there is soil and things grow really well because there is a lot of rain.