Re: Will one 18U college player put you in Major D
Since you mention "Major" I may not be thinking of the same thing you are, but I think some of the top showcases don't allow college players (Aurora?), but otherwise I don't think there's a problem, as long as she's age-eligible. Our 18U team certainly played (ASA and NSA) many teams that had players with a year of college softball experience.
There seems to be a building sentiment that they should not be allowed - at least for showcases.
OK, one more edit - it seems to matter in USSSA: "18U Division will be played as an "open" division. USSSA distinguishes between 18U teams with one or more players attending college (18 Majors) and 18U teams with no college players (18-and-Under) ... Any berths won by an 18U Major team in an open tournament would be to the 18U Major World Series only." More here:
http://usssafloridayouth.com/florida_usssa_girls_fastpitch_softball_rules.htm