Multi Ch. speaker placement tips needed

Hello all,

I’m initially setting up a 5.1 system in a < 14ft x < 21ft < 8.4 room. Closed off 100%, save for AC ducts. The ceiling gently slopes up to 8.4 then back down, to about 7.9 ft.

All the speakers are Silverline Audio… Sonata IIs; Center stage; SR 15; Velodyne DD 15. If I add to this config it will be a pair of Minuets for the middle surrounds and another sub for the rears… that has yet to be decided though.

Center surrounds will need to be set onto the side walls if done at all.

Center and mains are set to the < 14ft wall.
Where is the center to sit? On the same plane as the Mains? Or behind them? How high off the floor?

The rears are on Sound Anchor stands and the tweeters are right at ear height, 48 – 80 in.

If I go ahead and add the senter surrounds should they also be set as the rears in height?

Higher? Making for a hemispherical config?

Lower? Making for an increasing slope up to the rears?

Thanks much for your experiences & interest.