I got a mini skirt for my birthday party for few more weeks, which is about 5 inches above my knees. Is it too short a mini skirt to wear thigh high stockings with the mini skirt? I mean, does it matter whether the stockings tops show a bit if I am not careful when sit, etc.

Well, if your stocking tops showing, you can be considered whore-is.
I am sorry; I am not saying you are. But I would go with tights or stockings that start at your waist, the body stockings or pantyhose.
5 inches is probably OK, if you do not have short legs. Make sure the tops of your thigh high are long enough to not appear. Or you can wear leggings, or perhaps high socks (kind of like what they did in the year 1970).