when are we supposed to include "to/ta/ten" in a statement like this? Wouldn't "Jsou malá zvírata" also work? or would it mean something different?
The only case I can imagine you could use the sentence "Jsou (jako) malá zvířata." (They are/act (like) small animals.) in is when you would be talking about a group of kids that act really bad (like they don't behave and they are rude and scream and fight with each other), so you would metaphorically say that they act like animals.
"To" in the sentence "To jsou malá zvířata." means "These/they are small animals." - it suggests that you say which animals are small.
If you wanted to say the sentence "The animals are small.", it would translate as "Zvířata jsou malá.", and that would be a general statement about animals being small.
"To" is subject in this sentence. And subject can't be omitted here.
To - subject jsou - verb zvířata - object malá - modifier
What about in the context of a conversation. For example, when two people are talking about some mice and the way they behave and one of them wants to say, referring to the subject of "the mice" (maybe some specific ones, or maybe just a comment about the genus in general), "Well, they are small animals."
What form would such a sentence take?
Because we have not plural for "to" . When we want to use it in plural we have to use verb in plural instead.
Yes, as a single word it would. But in sentences when you want to say It is/They are XXX. you use:
singular - it is - to je (for all genders)
plural - they are - to jsou (for all genders)