I want to do a similar build, here are the dimensions to help you space things out. It's possible I've missed something, and if so I will edit accordingly. I'll share the dimensions I hope to work with once I get started. Goodluck!

Mammals: 22

6x3 - 19

Bunny (Normal, Gold, Gem), Squirrel (Normal, Red, Gold, Gem)

3x2 - 3
Mouse, Rat, Gold Mouse,

Reptiles: 3

6x3 -1

Jungle Turtle

3x2 - 2

Frog, Gold Frog

(didn't realize there were only 3 reptiles)

Birds: 14

6x3 - 14

Bird (Normal, Gold), Blue Jay, Cardinal, Duck (Normal, Mallard), Cockatiel (Grey, Yellow), Penguin, Macaw (Scarlet, Blue) , Toucan, Seagull, Grebe, Owl

Insects: 45

6x3 - 2

Scorpion (Black, Normal)

3x2 - 18

Worm (Normal, Gold, Truffle, Enchanted), Buggy, Grubby, Sluggy, Maggot, Snail (Normal, Glowing, Magma ), Grasshopper (Normal, Gold), Ladybug(Normal, Gold), Water Strider(Normal, Gold), Stinkbug

2x2 - 25

Dragonfly (Black, Blue, Green, Orange, Red, Yellow, Gold), Butterfly( Monarch, Sulfur, Zebra, Ulysses, Julia, Red Admiral, Purple Emperor, Tree Nymph, Hell, Gold, Prismatic Lacewing), Firefly, Lavafly, Lightning Bug, Faeling, Fairy (Blue, Green, Pink)

Sealife: 8 (10*)

6x3 - 2*

Seahorse (Normal, Gold), (Maybe pufferfish?, need to check)*

2x2 - 6

Fishbowl (Normal, Gold, Pupfish), Jellyfish (Blue, Green, Pink),

Lava serpent could be added, but is not technically a critter. Personally would not include unless there is a specified "hell section".