I'm not sure if the weight matters so much but if it does get chubby and you're worried about it just keep the light off and feed your pet only non-fat till its weight goes back down (or a low weight potion)

When I got my Mayonaka finally the magnifine glass had told me it was gonna evolve into a Piero, so don't put all your trust in that.

I'm glad I got Mayonaka cuz it was the last spotted egg I needed and it was cute but I was disappointed that it had no quests

Yeah basically you just need to keep your lights off so it won't evolve, and work on getting it maxed discipline. And then when you get it maxed in discipline don't automatically turn on the lights, give it a little longer like a couple hours and make sure you keep its happy/hunger levels full

Sometimes you can pick up and throw the veggies (or your pet) right before your pet starts to eat it, and when it lands again there's a chance that your pakka will then reject it. Sometimes you have to throw the food around the room a few times before they try to reject it. Also as for foods; mushrooms, carrots, and veggies in general are the most rejected but I've also had my pet reject rice, yogurt, bread, and cheese. Good luck