@Loone

You can convert pumpkins into food on the Pixpet Food page, found under the Pixpets tab. You can also convert pumpkin flowers and sprouting pumpkins into food, although they'll give you less food points.

As for the second question, this is mostly a guess based on my beta experience, but the reason other people's pets have more levels despite having the same number of hoards as yours could be because they've been using all their starter potions, and potions seem to fetch more HEXP than a normal hoard (usually). But someone please correct me if I'm wrong. Edit: It could also be that they simply have a higher SLVL than your pets, and/or better luck! ...but I'm leaning towards potions.

i would say it's definitely the potions that fetch more HEXP + a combination of just luck/SLVL or different pots used at the same time. i used up all my starter potions on my canisqua and he's already so beefed out he's close to needing his evo bar

saving up your potions is definitely a good option though, but they can be created later via recipes if you're worried about never getting the chance to use them again!