Let's see if I can explain it a little bit better.
I hate to be strict, but I think a few guidelines should be used for the Olympics, as it's a special event and there should be some differences to add a little bit of variety.
Players who compete as a team at the Olympics must be from the same country: we won't see Ruano-Pascual / Suarez in Athens, and I think it's fair to do the same here. So, answering carot's question: you can play the Olympic doubles with The Crow only if, as you suggested in the other thread, he changes passport switching from Belgium to the Mean Admins Team Formerly Known as The Republic of Kouklara; or, of course, if you play for Belgium.
Said that, the regular doubles tour and the Olympics can be played with two different partners. I'm sure many tennis players will do it next year.
Replying to Valda: Belgaria sounds great!
Fantasy countries are welcome. If you and Tim represent Belgaria at the Olympics, you won't be able to play again for Ponderosa Posse in the next year's Fed Cup; but you and Tim may play also Fed Cup for Belgaria, as two players is enough to create a Fed Cup team.
While in the WTA Tour, playing Fed Cup is a requirement to be eligible for the Olympics, Tennis Poker Fed Cup is not compulsory in order to enter the Olympic tournaments.
Another note: there isn't any limit to the number of contestants for each country. For example, there might be 14 singles players and 7 doubles pairs representing USA, that's not a problem: the top 3 players and pairs will enter the main draw directly, while all the others will have to play the qualifying round. That's not related to Fed Cup at all.
If you have any doubt I'll be happy to answer your questions.