I switched a Gauss Rifle after I was unsatisfied with builds using the default AC/20. Decided to squeeze in a ERLL for the heck of it, and discovered something surprisingly effective. The Gauss rifle and the ERLL have close optimal ranges, so I feel they are well paired for fire support. The ERLL is even head neutral; great for when using the Gauss rifle is out of the picture. As icing on the cake, it has a MPL in the zombie slot. Unfortunately, there is very little room to tweak this guy; I hit max tons and max slots, making it very difficult to cut somewhere to add somewhere else. The only thing I could see myself doing is dropping the zombie slot for a LPL or ERPPC
All I have are minor tweaks. Move two armor from the head to the CT, move a ton of ammo from the RT to the CT, move a DHS to the RT to finish padding the Gauss Rifle. I wouldn't worry about the ERPPC. You have good pin-point damage with the Gauss. The ER Large will keep the pressure on anything that you can't get a clean shot at with the Gauss. You've gotta move with other 'mechs in this build. You're far too slow and vulnerable to stray at all.