First off, welcome to The Cat Site, mymeeshell!
As for food! Fancy Feast Classics and Wellness Core are all grain-free. Overall, if you look at the food charts on Catinfo.org (PDF here), you can see that FF Classics are lower carb (I think in all cases!) than Wellness Core's flavors. As Dr. Pierson notes on Catinfo.org, grain-free doesn't necessarily mean low-carb.
As to the specifics of Wellness Core, I'm not sure if you're feeding Wellness's indoor or chicken flavor (I think it's the indoor, which just so happens to be chicken! ) so will mention things about both foods. The indoor food is definitely a higher-carb food, at 14% of dry matter. (Dr. Pierson recommends staying at 10% or below.) The indoor food is lower-calorie but higher-carb, and that's a mean trade-off. It's most likely the pea products that raise the carbs in the indoor food. Personally, I won't/don't feed peas for that reason. Since they're a carby legume, the peas would be my prime suspect for the smells you're smelling.
If it happens to be the chicken flavor of Wellness Core that you're feeding, well, that one has potato in it, and potato can cause digestive issues for some cats... including one of ours, who gets nastily gassy and a little barfy from potato.
If you're looking for a lower-calorie, low-carb, high-protein canned food for indoor cats, I might suggest one of Tiki's chicken foods (expensive but worth it -- our cats really like them and the luau food cans are packed with meat) or Weruva's Cats in the Kitchen canned chicken foods (our cats love their Chicken Frick a See). All those are shred-style foods instead of pates. Do your cats have a preference for shreds or pates? If your cats were doing well and weren't gaining weight on Fancy Feast, maybe it's best to just switch them back?