I think my preferred secondaries to go with beasts, are darkness, time, and Marine.
Darkness: all the usual great reasons to have this power set, including controls extra pet, debuffs out the yin yang, and the pets being all melee are reliably where you want them to be for the massive healing.
Time: virtually smorgasburg of a variety of powers packaged together, with Defense and most of the things also affecting The Mastermind being top of you. It's like an armor set for you and your pets.
Marine: like darkness, your pet's being melee mean they are in the right position to receive all the goodness that Marine offers predictably. Like both of the other sets, this one offers quite a bit of many different things that synergize well together especially for melee.