1
That's light. I recommend the reciprocating mass for an AR15 be around 10 oz WITH reduced power buffer spring. Your are 1-2 oz short with JP SCS with weights removed, but it can be done. F = m * A. if the mass (m) goes down, the A (acceleration) goes up and your BCG speed goes through the roof FAST. The faster the BCG cycles, the harder it is for the magazine to push a round in front of the bolt, and the more wear you get or your BCG and hammer. The solution then is to reduce F (force). F is your spring. The less spring, the less the A acceleration. The JP SCS offers 80 to 85% of a standard springs force AND it loses less energy to "sproing" like regular springs. This means a JP90 spring might run on less gas than a 10% reduced power standard buffer spring.

2
My 18" rifle uses JP SCS WITH steel weights on + ALUMINUN ULMOS + Hiperfire 24C + adjustable gas block. It is reliable when set to barely lock back every time. I used it for 3 matches with the gas set to load the next round, but not always lock back and I got burned under the clock doing that once with a short stroke after the gun got dirty. Since then I set the gas to lock back like everyone else. I have tried it with the weights removed and it jammed a few times. Mostly the rounds would fail to be pushed up fully in front of the bolt. The gun kicked no less than it did with the weights on, so I put them back on. The JP ULMOS does allow for a working forward assist that most steel LMOS do not. click here for 18" 3 gun build
(note: I have put my suppressor on this gun and it ran just fine. The RIFLE gas system covers a lot of sins up. However, switching ammo turned the gun into a bolt action. The gas has to be tuned not just for CAN vs NO CAN, but also each time you change ammo.

My 12.5" MIDLENGTH SBR uses the JP SCS with weights removed + JP STEEL LMOS + Hiperfire 24C + adjustable gas block + Suppressor. It don't jam except with Freedom Munitions 77gr (even then more gas makes it sing again)........... click here for 12.5 SBR for 3 gun
However, I never got it to run reliably with ALUMINUM carrier + Suppressor + ANY lightweight buffer combination ( plus it got dirty after 5 shots). It took AT LEAST the JP SCS with weights on to run. I never tried to get it to work WITHOUT the suppressor, but I bet bare barrel could be made to work. ALUMINUM bolt carrier + suppressor is a bad combination.

3
POF roller bolt cam pin is only necessary if using very weak recoil springs, but I like it. TITANIUM firing pin is a must with ALUMINUM bolt carrier like the ULMOS because the BCG is cycling faster than normal and slamming the firing pin against the primer. A soft primer + ULMOS + standard firing pin + stiff buffer spring might close fast enough to ignite the round.

4
It will work with the POF roller bolt cam pin IF...IF.. you are very good with a dremel or mill. You have to grind the carrier key to clear the installation of the POF roller cam. I have done this 3 times. The trick is to NOT take away too much material. Actually, one of mine I had to press the roller in and cant get it back out cause I did not quite take enough off. If I ever want to remove that bolt I will have to do more dremeling which I don't mind. If you remove too much material you will probably blow a hole in the key which could also blow a hole in the upper receiver right in front of your face. Don't screw up. An alternative would be to simply remove the Rubber city adjustable key and install the one from POF that is already clearanced for the roller follower. And last you could get a new set of gas key machine screws, remove the rubber city gas key, permanently install the bolt and roller follower, then put the Rubber City key back on with Loctite and new machine screws. I would consider the firing pin a permanent install as well so the bolt cam does not turn out of alignment. What the POF roller bolt cam pin does is make sure the weakest spring you can run will still be able to chamber a round and not have to fight the resistance of the bolt cam pin dragging the inside of the upper receiver. Also, I use the JP enhances gas ring over the standard 3 piece ring. With the JP enhanced gas ring + POF roller cam follower a steel BCG will close under gravity with virtually no drag. ULMOS + JP enhances rings + POF roller cam will close with a light push or jolt.


Your build should work, but the low weight of your reciprocating mass means you may have to adjust the gas block for different loads which means the Rubber city adjustment would be useless. You will need an externally adjustable gas block.
-Your Rubber City Titanium BCG
-Replace Rubber City Key with POF key, new bolts, Loctite key on BC, Loctite and re-stake new bolts
-POF roller cam pin
JP enhances gas rings
JP titanium AR15 firing pin
-SCS with weights removed and JP80 white spring. If you cant get the gun reliable, just put the weights back on and readjust the gas
-Hiperfire 24c or any other trigger equipped with a JP Yellow hammer spring. You would have a titanium firing pin so the yellow should ignite anything.
-externally adjustable gas block = SLR, Syrac Ordnance, or that new bypass model.
 

Quoted:
Maybe take a look at the Magpul SL-K stock?
I have two "light weight" builds with them and really like them.
Just a thought...