Hi JD,
1- XP uses simultaneous phases, so the proper answer would be "100% requirements, , 100% design, 100% coding, and 100% testing."
In the book, we recommend that you have two people focused on requirements ("on-site customers") for every three programmers. We also recommend one tester for every 4-6 programmers. That leads to the following ratios:
36% of the team is dedicated to requirements full-time. 54% of the team is dedicated to designing, coding, and programmer testing full-time. 10% of the team is dedicated to testing full-time. The ratios are just rules of thumb and must be modified for your specific situation.
2- The book is written for teams as small as five people (four programmers and a product owner). You can go smaller, but some practices would need to be changed.
3- You can always modify your approach to agile development. However, you're correct that communication is the key to success (in any project!) Agile methods prefer face-to-face communication over document-based communication. It's one of the reason's they're successful. If your customer isn't the collaborative type, you'd need to figure out some way to improve communication.