Focusing on the rest of the model for now sounds like a good idea - the tail can be a thing for the future, once you’ve decided on what style you prefer 
The tail would make an excellent addition to the expression of feelings; however, remember that there are (for now?) only 5 states which would have to be expressed: Forlorn, Glum, Content, Merry, Jubilant.
Now, I’m not a cat-person, and I have no idea how cats “work”; but based on the handy table you provided, @Bella_Chatlotte, I’d say that 1 or maybe 2 “animations” is enough for each of the five feelings stated above - except possibly for Content.
When it comes to “ease up” the animating-process of a tail like this, I’d say the imporant things to identify is “how does a sad cat-tail look?” and “how does a happy cat-tail look?”.
If it is possible to identify what the happy and sad states look like, they can be used for Glum respectively Merry; further more, they can be “exaggerated” for Forlorn respectively Jubilant.
(For dogs, a sad tail is typically a lowered one; a happy tail would be raised and/or wagging one - both states are simple to exaggerate)
For now, a style of tail could/would be the main focus as far as the tail go - and the additon of having it moving about can be added in the future, yes.
I’d like to say that it should be somewhat easy to add such a feature down the line; I’m fairly new to modding, but I can’t think of any reason why it would be hard to add.
I never really stated my idea about how a cat tail could look, but when you started talking about animating it this is what I pictured (again, as an animator/modder, not a cat-person):
A “Tail Section” is nothing else but a 2x2x2 Voxel Cube; combined desired number of these sections make the Tail.
Let’s say we have 4 sections simply stacked on eachother - a simple animation (in a 2D view) could look like this
(Sorry for the crude example; I’m just about to leave home and wont be back for a few days - wanted to provide my feedback prior leaving. Please, try to picture these squares as cubes that are moving only on the y-axis)
This animation cycle could be looping; mirrored; expanded, etc - all pretty simply.