Other than going for identical high quality speakers all around, you want to spend more on the LCRs. With app. $1000/channel, average, you can do quite well.
As for the elevation of the surrounds, place them so the tops are 70~72" above the floor. If they were 24" tall, as mine are, then they will clear the tops of the seat backs. My preference would be to use the same speaker for all 4 surrounds.
As for the LCRs you are going to have to make a decision between using identical towers or stand speakers or using a matching center, non-identical.
As you are going with a screen, if you use an AT screen, you can then place the LCRs behind, using identical towers.