Also, I've found that I never use gauss weapons in FMP. Ever. By the time I can research them I already have enough elerium that I go for laser weapons every time, especially since gauss and laser weapons are researchable choices at the same time.
With gauss, although there's definitely power and weight and firemodes on its side (and the silly 8bitish sounds are cool), lasers are just so much better. Entirely due to the damage type, because you never know what alien type you're going to face before you land, and lasers are all around good vs everything, whereas gauss will wreck some enemies and be completely worthless against countless others.
So, my thought is maybe move gauss a bit up the tech tree? Maybe put them as a result of the advanced firearms tree + some alien ufo research?
That way, you start with what xcom starts with, work up to autocannons and such, down a few ufos and a terror mission, and do research that doesn't require interrogation to get gauss.
I know it doesn't make sense conceptually because of the power source limitations, but gameplay wise It's a significant upgrade in firepower that works on early aliens but starts to become severely hampered as you go along due to damagetype resistances.
Alternatively maybe add explosive/incendiary for the starting conventional weapon set as well once you research it? I've really found that the basic assault rifle becomes my mainstay until I get dart rifles/lasers, because all the other options have problems with accuracy or lack of autofire or weight or such.
I guess I could try experimenting with shotguns, as I have usually avoided them, but I've noticed that aliens use them at extreme range with incredible efficiency..
Especially because of the way the projectiles work.
First couple times I played FMP, I'd go multiple turns with an alien shooting a shotgun at my team and not knowing until people keel over dead or start bleeding out for no good reason.
Then I find theres some alien on a skyskraper with a caster who's been taking potshots, but not showing up on alien turn, explaining all the random potholes that keep showing up.