The strength requirement is so you can't have minigun totting troopers. Not until a months or two into the campaign, where fighting alien and sniffing plasma turns your scared newbies into competent shock troopers. It makes perfect sense.
Yeah tottal perfect sense, because even today in 2015 we have minigun totting troopers using earth science... M8 do you know what will happen to a human if he fires a chain gun like this from the recoil alone? There is no way even a strong human can fire the damn thing without servo assistance and stablilizers and remain as he was. And earth science hasnt made these practical yet asside from fixed platforms or mounted on craft/vehicles. Why would they be in the games time frame without extra research been used by a single guy without assistance?
As to the second part (Not until a month or two). Why would you use a minigun if you have an autocannon with three types of ammo and arguably less requirements to handle or even more exotic weapons dependent on how well you did*? My case rests on making the minigun a decision based on effectiveness and cost as to opposed to high STR no brainer, that you have access from the start (which you wont use later because you would have better options, how many people used the heavy machine gun? I tried but the requirements make it practicly useless).
Hence my alien alloy requirement suggestion (mitigates in theory the recoil and lowers the weight making it able to be handled by a strong guy) and a slight boost to damage to compensate for the matterial and time spend on research. So to sumarise: Experimental weapons + Allien alloys -> Heavy chain guns = Minigun + Light minigun (Had it be on me I would put the auto cannon here as well). IMHO it will make a costly heavy weapon which will have a balance between power output, weight and been able to be used as opposed to forgotten. And having to spend time to research and matterials to build those will make it a desicion as opposed to a freebe for a high STR guy.
The final answer will come from Solarius anyway. This is his mod hence his vision, I am just throwing arguements/ideas.
*To clarify something: I am not the breed known as a power player, and I do understand the need for additions for enjoyments sake. I really do, but I would like to see everything having its practical uses and tradeoffs. Even if its just for fun.