What about optionals as "μιλά" and "μιλάει"? Should they be added or is the former colloquial?
yes, I'd say "colloquial''. While it's fine for the spoken language in a course we'd want the more formal version. There's not enough space to include much colloquial, regional etc versions of a language.
how does σiγά difer from σιγά σιγά; what type of meaning or emotion or sentiment does the repetition convey?