Quibble: You said Ostsol 2Cb, when I think you meant Ostroc 2Cb. Likewise, the PXH-2c "Special" might be a -1c "Special" instead.
As a force balance, that seems plausible, but tricky to be sure of. The RWR mechs are more lightly armed, especially at long ranges, but more heavily armoured. I suspect the SLDF will be playing matador quite a lot if they can, with all those ERLL and ERPPC floating around, so you'll want to be careful how you design the map. If it's too open, the SLDF will dance at long range and massacre the RWR forces, as they have neither the long-range firepower to compete nor the speed to catch up. If it's too enclosed, the armor and close-range firepower of the RWR forces will dominate.
Here's a quick list of every weapon on both sides with more than 15 long range:
RWR - PPC x16, AC/5 x18, LRM-15 x8, LRM-10 x4, LRM-5 x4 = 430 max damage
SLDF - ERPPC x4, ERLL x24, Gauss x4, UAC/5 x4, LRM-20(A4) x4, LRM-15(A4) x16 = 652 max damage (assuming Lancelot and PXH-1c)
Also, you're going to need to be careful what you do with those mines and Castles Brian. Some amount will definitely help level the playing field, but the details will matter a lot. Too many minefields and the SLDF will be sitting ducks(either through fear of mines or through leg loss), and a Castle Brian is a hell of a hard target to break down. Were you thinking of giving either side any conventional assets to go along with this, or any artillery? Arty on the SLDF side will probably neutralize the Castle Brian somewhat, since it's not like it's mobile. Is the CB going to be packing long-range weapons, and if so how does that affect the battlefield? Is it going to have anything monstrous like naval weaponry that can be used against ground forces?
This seems like a very interesting starting point, but I'd proceed with some caution.