As of now, the limitation described by Draloch does not apply for technical reasons.
If you have 10 characters and go F2P, the 10 are all available for you to pick and play. There are currently no mechanism that makes you choose 2 (and soon 6 for privileged) active characters out of your entire roster.
Blaine Christine mentioned that "in the future" EAware will put the restrictions in place but with no date.
See F2P Q&A for source: