Here's some additional information from Joshua Wood regarding Francisco Rodriguez:
According to mlb.com, KRod was added to the 40-man roster on 9/15 (if I'm reading 'selected contract' properly as synonymous with 'purchased contract.') This makes the unnamed Gammons source wrong, and the move perfectly legal.
Purchasing the contract does indeed imply that he was not on the 40-man roster. That's what the term 'purchasing' means in this context. And the Angels did make a move to clear a 40-man spot a week earlier:
09/08/02 Outrighted off 40-man roster Elpidio Guzman (OF).
KRod was signed in 1998, when he was 16. For players signed when younger than 18, they get 4 rule 5 drafts where they don't have to be protected on the 40-man. KRod's 'free' drafts were 98, 99, 00 and 01, so he would have had to have been added to the 40-man this offseason in any case. Despite Scioscia's toying with the idea of calling him up, there's no reason to move him to the 40-man until necessary, since you have to outright somebody off it.
Being on the 40-man rosters *has* to have bearing on playoff eligibility. You can't play in the majors for a team if you're not on the 25-man roster, and you can't be on the 25-man if you're not on the 40-man.