When we use pretty with please as on “pretty please, then “please is used as an intensifier, right? Can’t we use any other intensifier with please?
No. In that phrase, the request is please and its intensifier is pretty.
Please? Pretty please?
Especially among children, this can also be followed with:
Pretty please with sugar on top?
The only other intensifier I can think of in combination with please specifically is something like this:
What can I ever do to please you?
Other intensifiers with other forms of the word include saying that something is very pleasing or that someone is not too pleased.
Here is another idiomatic expression where pretty might be considered an intensifier:
That's a pretty pickle you've got yourself into.
(Which is similar in meaning to "another fine mess," even though no intensifier is used in that expression.)
But in general, pretty is not thought of as an intensifier.
This is a fixed expression and not productive. We generally can't use other intensifiers with "please", and we can't say "pretty thank you" either.
We also don't use "pretty please" when please is part of a sentence: We don't say "Pretty please tidy your room". We can strengthen the request to a demand by omitting "please": "Tidy your room!". Or soften it with tentative language "Would it be possible for you to tidy your room?"
There are a number of joke expressions like "please with a cherry on top" that are probably best avoided, as they are not very funny.