I'm playing Pets Live and I often discover new Pets
But is there any advantage to upgrading these or should I just focus on my main four Pets (Water, Earth, Air & Fire) and keep those maxed out?
Only your 4 strongest monsters are used in combat, so it's only beneficial to maintain 4 pets.
Of course, there are better pets to be unlocked than the ones you start out unlocking. The best pets are unlocked around level 85, and the ones you use will depend on whether you want to be an attacker or defender.
For more information: see this guide