Rajouter has two main meanings.

• Add again another instance of something already present: Il faudrait rajouter une assiette à table.

• Add later something to some other thing(s): Si tu oublie de rajouter la levure à ta pâte, elle ne lèvera pas.

Both meanings clearly overlap and they overlap too with the simple ajouter so ajouter and rajouter are almost always if not always interchangeable.

In your example, rajouter has both meanings: add again an item to the list of characteristics, and add later a different characteristic to the list. Using ajouter is also possible but would slightly reduce the idea of repetition and accumulation.

About "croiriez" vs "croirez", if rajoutez à ça is considered a conditional clause, as it is using the present the future is the expected tense in the main clause:

Si vous rajoutez à ça une voix haut-perchée, vous croirez entendre…

Should the sentence had started with the imperfect, the conditional would have been required:

Si vous rajoutiez à ça une voix haut-perchée, vous croiriez entendre… *

However, the sentence is not exactly built that way, the first part is more a statement than a condition so I believe both tenses are correct.

Using the future gives certainty to the conclusion while using the conditional makes the second part more hypothetical.