Please remember that it was a different company that produced that core book and mentioned it. And unfortunately, ICE has never been clear on how it should be done.
In RMC, ICE recommends that the extra Base Lists be limited to Open and Closed of the same realm, but in RMC, Base Lists are the only lists that are allowed to be learned above 20th level using the core rules..
I will try to give some guidance......
1) Hybrids do not get extra Base Lists. Caster types are Pure, Hybrid, Semi, Non. That rule says "Pure".
2) I would recommend that only Open and Closed lists from the same realm be allowed as extra Base Lists. But which lists to allow are specifically left up to the GM to determine.
3) In RMFRP, the only reason for selecting another list as a Base List would be to change its cost as none of the other benefits from having a "Base List" are any longer present in RMFRP (i.e. the ability to learn it above 20th level).
4) When a list is first learned (i.e. the first ranks are bought), that is when it must be decided whether or not it is a Base List.
5) That second ruling -- what I think it means (remember, that ruling is from the time of the old ICE) is that certain races are granted access to certain Open spell lists as part of their Adolescence training. And that, if the player wants, he may select that list to be a base list without having to repurchase those ranks again.
I should add a caveat to this. Any such adol list, if later developed as an Open list during normal development should then be disqualified as a candidate for being an extra base list.