> I read somewhere that, although the 'correct' way in this case is the first one (хороша,) it is normal to use the long version (хорошая) instead in speech.

"Хорошая" is equal in rights with "хороша" here. Yes, it is absolutely normal, and moreover, because "хорошая" is more universal it tends to be used more.
However there are differences at some cases. (Not in this case.)
For example, if you meet lost child and want to seek for his parents with him, you can say "я - хороший" ("I am good (man)") to calm him down. On the other hand, if you make good move in the game with friends you can exclaim "я - хорош!" "(Damn,) I'm good!". It will be mistake to misuse these forms in these cases.
For beginning you may think that short forms "хорош/хороша" are more rare and because of it - are more valuable". Some sort of little bravado or excitement. "Девушка хороша" = "Girl is good/beautiful". "хорошо" is another case ("мне хорошо" = "I feel good").
"Книга хороша" is very ok and have a little bit of excitement.

> What I meant was that I thought imperfective 'плакала' (meaning 'I was crying') was chosen rather than the perfective form, in order to bring attention to the crying itself, rather than just the fact that she had cried.

In russian "я плакала" is equal to "I cried" too. There is no special form for "I cried" in russian.
To say the truth "rules of present/past/future" is most difficult part in learning english for me. Because we have another ones.
As far as I know:
"Я плакала каждый день" = "I cried every day"
"Я плакала два часа" = "I was crying for two hours".