My observation is that you can only train a cat to do things it pretty much wanted to do in the first place
That said, I
have "trained" various of my cats over the past 20 years to go out on a leash (mind, the cat has to choose the direction and pace - well, ok, I did have one who would pretty well 'heel', but she was overall a cat in a million), and to jump up where/when I indicated, and to fetch, and to stand up and 'pose'.
But this was all done purely for
fun, just building on what the cat was naturally doing. Didn't use a clicker, just praise and petting, pretty much just shaping what the cat offered. (e.t.a. - and some of my cats were not interested in walkies, or 'up', or 'fetch', or whatever, and so they didn't do it and I never bothered them about it again after ascertaining that it was not on the cat's agenda
)
Training a cat to do an arbitrary (to the cat, anyhow) task would unquestionably be much harder.
OTOH there
are 'cat agility competitions', a la dog agility, and while the footage I've seen suggests that agility dogs need not feel threatened <vbg> it would seem to indicate that cats *can* be clicker-trained to do relatively arbitrary things. Some cats anyhow. By some people.
So I dunno. It's possible. They're not one of the animals that most easily lends itself to training (clicker or otherwise), though
And six month old kittens are not exactly noted for their attention span
Pat, now really missing my long-gone Hecate kitty who used to walk with me outside on a leash and all sorts of other neat stuff