This is the way it works practically:
Frozen ATPL or CPL/IFwill allow you to fly on any aircraft as P2. If you get a Job on a FAR 25 aircraft or SFAR41 (eg. B1900) you fly 500 hrs do the ATPL test then when the company wants you to upgrade and your ATPL will be unfrozen in the process. Nobody will probably give anyone command on a FAR 25 aircraft anyway without sitting at least 500hrs right seat - so don't worry about it, it does not affect you - there is no need to hire a FAR 25 aircraft at your expense at all.