I know how a fire team is composed of an how pairs are often a rifleman and MGer and rifleman/grenadier and rifleman.

Generally, what are the chances of two Special Force Operators going on a solo mission armed only with machine guns (in this case, M60E3s)? What are the pros and cons of such a weapons setup, OR should I stick with one MG and the other with a rifle/M203?

Thanks.

When you say "solo mission", d'you mean just the two of them? In such a case I'd say it's unlikely that either of them would have an MG, unless they were being specifically sent to set up an MG to provide fire support. In which case it's more likely only one would have an MG.

Yes, just the two of them.

I figured that since the M-60E3 can be fired from the shoulder, and quite accurate, perhaps two SF Operators may have the weird idea of using them as "substitute MG rifles."

Thanks. I'll plug in a rifle for one figure.

Not just the M60E3, but most big GPMGs like the M60, FN MAG and PK can be fired from the shoulder (though you need arms like Rambo to do it for long). Over here, machine-gunners often carry at the shoulder like a rifleman when crossing open ground or assaulting so that they are less noticable to snipers.