The question of how many hands a weapon needs is left to GM fiat/common sense.
Technically, a Katana is a two handed weapon, but artwork going back to 1e show street samurai running around with a katana in one hand and a gun in the other... so the rule of cool is well entrenched in this example.
Ask your gm. Generally, I'd say that using a rifle one handed either means you can't gain edge, can't spend edge, or counts as a factor towards the other guy's favor in the circumstantial leg of the Edge triad. In the situation you describe where it's modified/balanced for one handed use, I'd probably say that last one if I were your GM. It might be lost in the wash, but then again that partially-compensated awkwardness might be the factor tipping the scale to edge for the other guy if everything else balanced out... I might even say no penalty at all if you're as strong as you are big, as the metahuman-adapted rifle could be to you what a pistol is to a mere human... OTOH if you have a human-like STR value I might say the recoil is so much that even if it IS sized and balanced appropriately for your frame you still can't control the recoil one handed, and go with a more severe penalty like you can't spend edge.