Re: pick a sentence..
Hi navigator,
navigator wrote:1) It's a feminine / female atmosphere.
 
This depends on what meaning you want for the sentence.  Using 
female is trying to say more clearly that the atmosphere is 
not masculine.  If you use 
feminine then you are either 1) trying to say that the atmosphere has female qualities (umm, I don't know, caring? understanding? etc.) or 2) trying to, with fairly bad grammar, trying to say that the word atmosphere is a feminine noun (as used in foreign languages).
navigator wrote:2) I knelt down / knelt / genuflect to see what was under the bed.
 
I personally would go with 
knelt down because it's more descriptive than 
knelt.  
Genuflect is most commonly used in instances where the kneeler is kneeling in worship, even though technically it could work in the sentence.  The difference between 
kneel and 
kneel down is minimal and choosing one over I would say is purely subjective.
navigator wrote:3) I found a sign.It had its meaning / tenor / significane.
 
Here I would rule out 
tenor only because it's not that common (I've never used it hehe) and would choose between 
meaning and 
significance by deciding how important is the meaning?  I see 
meaning as just basic meaning, if that makes any sense.  But when I say something has 
significance, I'm trying to say that its "meaning" probably affects someone or something.
navigator wrote:4) The room was overly-tidy / over-tidy / too tidy.
 
Here, I don't think there's a hyphen between 
overly and 
tidy, just because 
overly is just an adverb modifying the adjective 
tidy.  To me, 
over-tidy sounds a little sloppy, so it's not as impactful in speech in my opinion.  The other too are basically the same, but I would go with 
overly tidy because it sounds more ... impactful, I guess, more impressive.
navigator wrote:5) This is a lady's boudoir / private room.
 
Since 
(lady's) private room is basically just the definition of 
boudoir, I couldn't decide on that alone (and one could argue that 
a lady's boudoir is redundant).  I would instead decide if I felt the audience to whom I was speaking would be familiar with the word (adults vs. middle schoolers, men vs. women for example).
Hope this helps!