I'm not really very clear what you are asking here, but anyway:
Finding the right key for YOU to sing a song in is absolutely essential.
Everybody's voice has a sweet spot. You want the melody of the song to be in that sweet spot.
So take the chords and play them with the capo everywhere until you find a place where your voice sounds best -- you can hit both the hight and the low notes. You'll know this key when you find it.
Now you can choose either to play the chords as they are with the capo, or transpose the song so you can play the same chords with the capo in a different place so it sounds better.
For example, suppose the chords in standard tuning without a capo are E, A and B.
You play around and your voice sounds best with the capo at the tenth fret. So the chords you are actually playing are D, G and A.
You can choose to play them with the capo way up at the tenth fret, but the guitar is going to sound really high, so you might decide to play them on the open strings, using D, G and A shapes, or somewhere else.
Basically, find out where your voice sounds right and then figure out how to play in that key on the guitar. Not the other way round.