You could use a batch process to ensure you're images and thumbnails are all
the same size.Just go to File > Batch Process and play with the options that
come up.

As for the first part of your question, you'll want to explore options in
either regular HTML, JavaScript, or Flash. A third party program would most
likely save you plenty of headache. Search for relevant keywords at
hotscripts.com.

Fireworks is a graphics program, and cobbling together some kind of rollover
effect with it to display your photos in a gallery is only going to create more
problems than it solves.