Might Stewie perhaps be an extrovert? He's rather charming, confident, funny and talkative to be an introvert, I feel. Of course, introverts can be all of these as well, but they keep to their own a bit more. Stewie initiates conversations and isn't afraid to say what's on his mind, ever, heck, he even blurts out his death threats sometimes. He always has little comments he isn't afraid at all to say. He seems rather energized by hanging around with people, such as Brian or other people he feels that aren't as competent and poking fun at them. ESTJ/ENTJ? Lois seems more an introvert than he is.
Also, aren't SJs a typically more complacent type, while NTs are more progressive? Stewie's hardly complacent, he embodies progression and isn't afraid to go against the status quo, which is an ENTJ trait, right?
Also, ISTJ are the most common type there is and I'm not trying to base it on this alone, but Stewie's a rather unique personality that I don't see every day, if at all (obviously he's a cartoon character, but you get my point).
SJ:
All Guardians share the following core characteristics:
Guardians pride themselves on being dependable, helpful, and hard-working.
Guardians make loyal mates, responsible parents, and stabilizing leaders.
Guardians tend to be dutiful, cautious, humble, and focused on credentials and traditions.
Guardians are concerned citizens who trust authority, join groups, seek security, prize gratitude, and dream of meting out justice.
NT:
All Rationals share the following core characteristics:
Rationals tend to be pragmatic, skeptical, self-contained, and focused on problem-solving and systems analysis.
Rationals pride themselves on being ingenious, independent, and strong willed.
Rationals make reasonable mates, individualizing parents, and strategic leaders.
Rationals are even-tempered, they trust logic, yearn for achievement, seek knowledge, prize technology, and dream of understanding how the world works.
Based on this alone, Stewie seems much more like a Rational than a Guardian.
Stewie hardly cares for being seen as dependable or hard-working, he's more invested in himself than in anyone around him. He does seem to pride himself on the fact of being ingenious and independent and it's to his discomfort when he feels rivaled, he likes being different. Guardians are cautious and humble, Stewie likes to brag and he can be reckless at times. Stewie is skeptical of all, but this can be associated more with a Rational than the cautious nature of a Guardian.
If anything, Stewie seems more like an ENTJ to me, he gets impatient with incompetence, he's bossy, charismatic, funny, confident, talkative, ingenious and the list goes on.
Stewie gets things done, he's well-focused, this is a defining trait between Te and Ni positioning. He has ideas and he realizes all of them (Te), while someone who leads with Ni would be a little more scatterbrained by definition, look at all of his inventions and he's had one main goal throughout the whole series it seems and it's killing his mother and it hasn't changed.