3 150'S if you have the space.
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"matching speakers is better for music and the center is better for movie dialog" What a bunch of BS! What's going to happen when music comes out the center channel and dialog out the left speaker? Movies are mostly music with some dialog. If you get the music right, the dialog should not be a problem.
Here is a pdf file on ATC speaker specs.
Matching all of your speakers in a multi-channel system is usually the best way to go. For the record, I am no big fan of center channel speakers. However, the C6 center, SCM 50, SCM 100 & SCM 150 all use the same tweeter, midrange and tri-amplification system. Essentially they will sound the same on the mid and top end. The only difference between them is the bass driver configuration, and thus the low frequency extension and spl output.
C6 32hz 115db Dual 9"
50 38hz 112db 9"
100 32hz 115db 12"
150 25hz 117db 15"
On paper it looks like the C6 matches up spec wise with the SCM 100. For your room size I would recommend the 100 for the front L/R and maybe the 50 for the surrounds. Depth of the speaker may be an issue for surrounds since your room is only 16' wide. I noticed ATC mixes in smaller models for surrounds, so I am not sure which way you are headed for surrounds. I would also recommend a 7ch system with 4 surrounds since your room is 28ft long. That's a lot of space between speakers for a 5ch system. If you want to run the left and right in plain stereo without a sub then the 150 would be worth consideration for the 25hz extension. As far as the center goes, screen height may dictate what will fit. I would have to see your design to make a recommendation. Without it, I would say put the screen about 36" off the floor and mount the C6 right underneath it as close as possible. I am a little concerned that the 100 or 150 bass driver might interact too much with the floor.