Isn't 'tjej' a word for girl? My Swedish friends say 'tjej' all the time in place of 'flicka'. Is that just because they're Scanian?
Yes it is, and it's an accepted answer here. I'd say it's used more widely than flicka, at least for slightly older children and young people. It's pretty old-fashioned to call a young woman flicka.
As for a direct translation, I would say 'tjej' more shares the meaning of 'female', as it is used for a wider age-span. You wouldn't call an old woman over 60 flicka, but even though it may not be common, I don't think anyone would raise an eyebrow for calling her tjej.
Actually it always make me raise my eyebrow. It feels really odd when people above 40 call each others tjej/kille., but it is rather common.
Kille is the male analogue to tjej. It might be more accurate to say that the translation of "pojke" is "little boy." In English, it is still quite normal for even a teenager to refer to the opposite gender as a boy or a girl, but in Swedish, pojke and flicka are definitely not used much when referring to any post-adolescent person.
Is c in flicka somehow fricative (not sure about the technical name), like flih ka not coming from the throat, but being made by the tongue?
Here's how I think it is, someone can correct me if I'm wrong. When a vowel is followed by double consonants, it marks that vowel as a short vowel, i.e. short 'a' in hatt (hat) vs long 'a' in hat (hate). However, 'k' is never doubled (might be exceptions), but instead when marking preceding vowel as short, k is instead paired with 'c' as 'ck'. Examples, Short 'a' in macka (sandwich) vs long 'a' in maka (spouse), short 'u' in ducka (duck; as in keep your head down, not the bird) vs long 'u' in duka (set the table).
True, ck is just a spelling convention. We don't write kk to mark short vowel and long consonant, but ck instead (except in our most common word och, where we use ch instead).
Det säger att man skulle säga bara flicka. Inte en flicka. Jag förstår inte varför är det så.
It says that you should just say Flicka, not en flicka. I do not understand why the preposition is denied, when it is mandatory in the grammar.