Pairing this with my headcanon that Grace would be really good at swearing if he actually used bad words, and Simon would be the most awkward curser alive if he didn't.
From what we see, Grace is frequently the sassiest bitch around, he just gets creative (staggering waste of carbon???) and uses filler words rather than actual swear words (bc being a middle school teacher will do that to you). But I also noticed that when he did swear, it was like riding a bike. When he said "I was full of shit" it wasn't awkward or like he was forcing himself to do something he wasn't used to, it felt natural.
Simon, on the other hand (heh), has a tendency to just say variations of the word "fuck" for everything. "Shut the fuck up," "fuck me," throwing a "fucking" into any sentence when he's upset. He rarely used any other swear words (I know he said "shit" at least once and he called himself a "dumbass"), which makes me think that maybe he didn't grow up swearing but had to learn later in life (probably in prison, let's be fr) and now it always comes out a little clunky.
So I like to imagine a reverse au would also swap their swearing habits (bc that's funny!!). Simon ran a robotics club for kids and doesn't swear anymore, but instead of wasting brain power on anything other than engineering, he just slaps a "frick" over any bad word he wants to say and calls it a day. "Oh frick." "Are you fricking serious?" "What the frick."
Grace, meanwhile, would have the mouth of a sailor that regularly slips into borderline prose. Still cold, still calculating, well into the habit of biting his tongue, but if pushed far enough? Oh honey, he'll just go off.
So in the fallout of the aforementioned incident -
Simon: Hey hey hey- will you just fricking listen to me!? We're not going to kill you! Maybe take away your access to chemicals for a while, but-
Grace: Bull-fucking-shit!
Simon: Can you please just trust me for a second!?
Grace: Listen here you sorry excuse of a scientist! Life has been fucking me raw with no lube since the day I got spat out of the womb! Now all of a sudden I wake up in a biodome with Tweedledee and Tweedledipshit telling me everything is fine!? Oh, you're just gonna feed me, clothe me, house me, let me put actual fucking poison in your breakfast, for nothing in return? Like hell am I believing that! You're keeping me alive for some kind of sick, fucked up experiment, and until I find out what it is I'm not trusting your fatass as far as I can throw you!
Grace, of course, goes on to regret his outburst and doesn't speak at all for several days afterwards (monitoring the outcome), but at least now Simon knows what he's so worried about. Now how to convince an abused scientist that he's not going to be used as a lab rat?
(Grace also later regrets calling Simon a fatass. He didn't really mean it. He was just jealous of Simon's massive honkers. His big bad badonkers. His well-endowed pectoral paclonkers. His-)