We can do this using numbers, if you'd prefer.
1&2#:Let's suppose you have SoBx2, Finx2, and THx1, which are the stats that will affect weapons when you have MAc.
Assault rifle: (2d5+2)x3=12-36=24 damage, (0.8*0.7)=0.56 fire time: 24/0.56=42.86 DPS
Gatling gun: (1d6+2)x7=21-56=38 damage, 0.7 fire time: 38/0.7=54.29 DPS
Plasma rifle: (1d7+2)x7=21-63=42 damage, 0.7 fire time: 42/0.7=60 DPS
On the other hand, assault rifle also has +6 accuracy (which gives you ~75% at LOS range) while the other two have +2 accuracy (which gives you ~40% at LOS range).
3#: For Ammochain, you'll want at least one A on a plasma rifle to increase accuracy to 50% LOS range. The other two are useful but not nearly as much as having some higher hit chances.
4#: AA is typical if you can afford the extra agility mod. A lot of people invest in the second point of WK so they can boost the plasma rifle even further though, to (for instance) A1P2T2 or A2T3/A2P3. If you think you'll find the mods for it, there's little reason not to.
5#: High power plasma is a bit of a waste as mentioned previously. You're better off going the old fashioned way, at least for now.