I used dual Grim Reaper assault rifles for most of the story. Dual Silver Raven machine guns work well too, and are what I use most often. So the starting weapons are very much viable. Mixing and matching isn't often desirable due to differences in optimal range, though. Each gun has an optimal range where its damage is notably higher. For Grim Reapers, this is roughly 150 and for Silver Ravens it's roughly 100. You'll get a distance number for whatever you're locked onto generally to help gauge things. When you're at optimal range, the effects of the hits will also be larger and feedback in general will be stronger.
Really, a lot of the equipment in the game is more along the lines of being a side-grade than outright upgrades. Grim Reaper II and III boast higher damage and range than their predecessor, but have less max ammo and the increased range means more distance you have to maintain for optimal damage, for example. So things tend to be more situational for the mission or come down to what works/feels best to you.
