My team on Pearl beasts already and quite a few of them are low leveled.
Dragonite:
Level 80
Lonley Nature; Likes to thrash about
EV trained in Attack
Item: Expert Belt
Attacks: Dragon Dance, Outrage, Fire Punch, Earthquake
Gyarados (this one helped own a level 100 infernape):
Level 52
Adamant Nature; Likes to Run
EV trained in Attack
Item: Razor Claw
Attacks: Dragon Dance, Waterfall, Ice Fang, Giga Impact
Torterra (this one also helped own a level 100 infernape):
Level 57
Naughty Nature; Proud of its power
EV trained in attack
Item: Quick Claw
Attacks: Razor Leaf, Wood Hammer, Crunch, Earthquake
Staraptor:
Level 69
Rash Nature; Highly curious
Item: Shell Bell
Attacks: Aerial Ace, Fly, Close Combat, Brave Bird
Gengar:
Level 73
Bashful Nature; Quick tempered
Item: Spooky Plate
Attacks: Hypnosis, Dream Eater, Shadow Punch, Shadow Ball
Rattata:
Level 1
Serious nature; Good endurance
Item: Focus Sash
Attacks: Quick Attack, Super Fang, Assurance, Endeavor
Rattata helped me own my rival's level 74 Empoleon. XD It could have owned it, but then it wouldn't have been level 1. (frowns) So technically, I could have owned Empoleon, but I didn't want Rattata to level up. XD
About my Torterra and Gyarados owning a level 100 Infernape, I was battling my cousin who had a level 100 Infernape (that I trained!) and it was a double battle. I had already knocked out the rest of his pokemon and only Infernape remained. I'm pretty sure that Gyarados used waterfall and Torterra used Earthquake and owned my cousin's Infernape. (sighs) Fond memories.