Shotguns can go both ways. Charging the enemies and blasting them to pieces is aggressive and can be considered Renegade. Using shotguns for protection, to kill those who charge at you is passive and can be considered Paragon. Tali uses a shotgun but I don't view her as a Renegade character. Wrex uses it too and he does come out as a Renegade character.
It all comes down to the playstyle. I'd say that Paragon characters are focused on working in a team, using squadmate powers extensively and helping them survive with different debuffing and crowd control powers. IMO Renegade characters prefer going solo and focus more on killing enemies than on protecting those around them. Torfan and Elisum come to mind.
I view Tali as a Renegade character, albeit only to an extent - not as an overriding aspect (so I also see her as Paragon*). I think the look of her Purple main armor is appropriate - Red+Blue.
*This is different than someone like, say, Garrus, who I regard more as a 'tarnished Paragon', not a Renegade himself, no matter how he tries to be one or thinks he should be one. Garrus may be tougher, and be Omega Batman, etc, but Tali is the one with more innate and foundational Renegade attitudes about a bunch of things, even as it doesn't dominate her world view (she's one of the few Quarians who could have ended up allying with Legion, for example).
Other than that, I agree.
Paragon - Higher capability, shared strength with others
Renegade - Lower capability, concentrated strength
And since you're playing that Renegade, that concentrated strength aspect may be all you need. Why spread out your attention and care when you can more reliably secure your survival and victory by being a Soldier who punches everything in the face? 
(Just as a RP perspective, I mean. I'm actually a Vanguard who fights more at Mid Range overall and loves his Biotics and is Paragade in ME3 but more Paragon in ME1-2 - so I'm only explaining how a pure Renegade might view the combat situation.)
Then again, a Renegade may view others who prove their worth to be valuable assets worth some protection. It depends on 'how' Renegade you want Shepard to be. At the very maximum, yes, he is effectively set to be a Terminator with heavy weaponry that tosses everyone into the fray, and they understand that well enough because they know that this whole conflict against the Reapers is Do or Die anyway - they just may not necessarily like Shepard as a person for it. Not that a Renegade Shepard may care as much. But yeah, to make up for the feeling of a coordinated and supporting team, Renegade Shepard may be someone you want to deck in heavy looking armor/shielding, heavy weaponry, as a Soldier or close to it (Vanguard/Infiltrator.. maybe Engineer.. maybe maybe Sentinel for Tech Armor but ehhh... but almost certainly not Adept). Give off an appearance of combat style that screams individual power, instead of a closely in sync or supportive team. This is, again, a more Purist perspective though, and I'd actually suggest most players have some non Purist aspect to their custom Shepard anyway.