Regarding то...то. It seems to me there's a slight confusion here.

Мне то хорошо, то плохо. - in this sentence то...то must be stressed, otherwise it just doesn't make sense. Technically , то... то is a conjunction here.

But there is one more occurrence of то in Russian. This is a particle that is always hyphenated. For example, Он-то знает?. In this case то must be unstressed and used with hyphen, and it's something like He knows, doesn't he? (the usage of то here is for emphasis).

Another example is кто-то (pronoun, somebody), you can't stress то here, because it's just the unstressed auxiliary particle.

So, as it-ogo said this is not a preposition, but it can be different parts of speech.