Would this also work in a FR2? Or?
In an FR2 - no as it will not fit in the battery compartment as RC cells are larger than the AAs the FR2 is designed for. Any 12v external should work for the FR2 as long as you have the correct connector to fit the DC jack on the side. The FR2 will only work on 12v (I believe; I haven't tried powering my FR2 with any other voltage), unlike the 2LE which will work on any voltage from 6-12 (at least my 2LE works on 6, 7.2, 9, 9.6 and 12v).
For my FR2 I use a BatteryGeeks 10-14-88 which was superseded by a 10-14-100 which was replaced by a 10-14-135(I think it was a 135). Unfortunately BatteryGeeks no longer sells this style battery in any capacity. I "think" your best alternative would be a Naztech or Anker. If you already have a Tekkeon that would work for the FR2 also.