Don't worry about the balance of creatures to noncreatures, since some decks can have fewer of each as long as your synergies are covered
Merfolk have long been a supported kindred creature type, dating all the way back to Magic s first set Alpha via the O.G
You may well have to pick some things you don't want to and likely over things you did want and then shoehorn these random life gain things into your deck that you otherwise don't need or have better options you would rather have in the slot
And though it isn't one of the specific examples listed by Magic judges, card size differences could be considered a form of marking
They may play any card from their hand, unless that card specifies otherwise, providing they have enough mana to pay for its casting
The decklist includes just about every Squirrel and Squirrel support card you want to be playing, making it the perfect precon for those looking for a commander deck playing the furry creatures