Where can I find defeated quest enemies?

Most of what you are looking for can be found on the borderlands wiki

However, to answer the questions about your specific bosses:

• Skyscraper - It spawns under the separate broken highway against the far edge of the map, at nearly a straight connecting line from the World's Largest Bullet and Thirsty the Midget.
• Skagzilla - He's part of the mission Big Game Hunter in Dahl Headlands, and he respawns inside the pen area.
• Motorhead - Motorhead respawns every 15 minutes at the quest location.
• Mothraak - Mothraak respawns when you approach the torches.
• Rakk Hive - The Rakk hive is a special type of boss respawn which only happens after you've beaten playthrough 2. His location remains unchanged from the quest location.
• Crawmerax - He's always in the usual spot, he respawns when you leave/reenter the area.

Most of these bosses respawn when you restart your game unless otherwise stated.

As for XP gain, it entirely depends on your level. Farming bosses typically is not worth doing for xp, but rather for drops. This question is also pertinent if you want to get lots of xp.