and they pick who they want
Remember what happened in 2000? BJK picked the sisters over Raymond.
Not entirely accurate. 2000 was 'letter of the law'. What the rule said at the time was the three highest ranked singles players (Venus, Monica, Lindsay) and the captain gets to pick the fourth spot. That usually was the highest ranked doubles player. But BJK had wanted Serena all along, and since she was only 4th highest ranked American, BJK took her anyway, in the doubles spot. Which was not how it had been done the last three Olympics (Tennis only came back in '88).
Lisa Raymond screamed long and loud she'd been screwed. The arbitrator/judge determined that BJK had acted within the letter of the rules, though I believe he specifically stated she had violated the 'spirit' of them. So Serena remained on the team.
But no, they can't take 'whoever they want'. The top three singles players go. And there may have been a rule change about the fourth spot, owing to the controversy. But if Monica is one of the top three Americans, she'll go. And since she virtually doesn't play doubles, she won't get the fourth spot. Problem is, the Olympics are a year and a half from now. Will Monica be one of the top three Americans by then?
Lindsay, Jenn, Chanda, Alex and Meghann will all be taking runs at that spot, and I expect Lisa Raymond will hang on long enough to try for it as well. And Monica will be 31.