Doesn't look like a "kit hunt" thread to me - anymore than anyone elses threads seeking advice are. But here we are in the Restaurant. It would therefore seem appropriate that we start offering our body parts to readers. Oh well. That's perhaps not appropriate, no one fancy a nice rib? Particularly tasty if I don't say so myself.
Az, I wouldn't add a Meridian centre. As NutFlush said and I have to agree, keeping the tonality consistant across the front 3 is vital. Also, I wouldn't worry too much about the above and below centres. You have the speakers, so go for it. Run them both of a 557 and then when you're able to get a big B&W centre, you can bridge the 557 and you're away!
I'm sure it'll sound fantastic. Keep us posted.
Office: Meridian 500.1, 568.2mm, 557, 505,
Dali Helicon 800 L/R and C200 Mk2 Centre,
BK Monolith FF
Cinema: Valerion Visionmaster Pro 2,
Arcam AVR31,
KEF LS50 Meta LR, T300c,
Focal Dome On-Walls,
PMC Twenty.Sub
Bedrooms: Meridian 2x M80