The usual Latin diminutive suffix is -ulus (or -ula or -ulum). However, it sometimes appears as -olus, like in filiolus, aculeolus, petiolus, and bestiola. (And perhaps Venezuela, Venetiola, is a small Venice.) When is -olus used instead of -ulus? It seems that a vowel is followed by -olus and a consonant by -ulus, but I have not seen this rule explicitly stated. Does some grammar describe the choice of the diminutive suffix?
Background: I read recently about a children's parliament (which promotes children's ideas and teaches them the basics of democracy), and I wondered what I would call it in Latin. I thought a diminutive of curia would be a nice translation. Trying to attach -ula to curia produced curiola, as my intuition immediately rejected curiula. The problem is that I have no idea why I thought so. Therefore I would like to know how the vowel (o/u) is determined.