The diminutive ending -ula is common in classical Latin, and arguably productive.
The examples you found are not exceptional.
The linked question does not discuss all the Latin diminutives.
The suffixes listed in the question are all masculine, but there are corresponding feminine and neuter variants.
So the -ulus there implicitly includes -ula (and -ulum).
There are agent nouns for all genders.
For example, saltare gives rise to saltator, saltatrix, and saltatrum.
For more details, see this question.
The stem is revealed by the genitive form.
For my three examples they are saltator- (third conjugation), saltatric- (third), and saltatr- (second).
(The stem of rex is reg-, so it has a g instead of a c.)
If you ...
Are there any existing diminutives of agent (-tor/-trix) nouns?
Yes, though the rarest.
Examples for -trix have been already mentioned by @Joonas and @cnread: nutricula, meretricula...
I'd like to add an example for an adjective derived from a -tor word: punctatoriola, as the diminutive of punctatorius from punctator: the reference is Festi Fragm. e Cod. ...
According to Gildersleeve and Lodge, §182.1, ‑ulus is a regular former of adjectives from verbs. This particular formation indicates repeated action. They cite querulus and I could add bibulus. ‑bundus is unrepeated action (cunctabundus, nauseabundus). ‑bilis (amabilis, bibibilis, vendibilis) is passive action, as also, I suspect, is ‑ulis (edulis).
Even though a non-diminuitive version of credulus does not exist, it kind of has this "smallness" connotation. If somebody is credulus, he is childish in some way, so you call at least some part of his brain little developed. This naivity comes with a childish innocence and also dinkiness. Furthermore, this is meant derogatively, so there are some fulfilled ...