I have a Colt HBar II (A3) on which I would like to mount an EOTech 552 and ARMS #40 on the flat-top.  Will this work? Currently standard handguards.  

1) Enough room for the 552? (I know its longer, but realy want the AA batteries)

2) Will the ARMS #40 co-witness (centered) with the ARMS #40?  I have the A2 front sight.

3) Any interference with the 552, when the ARMS #40 is folded down?

Any suggestions would be appreciated.

Yes, Yes and No.

Yes, the 552 will fit perfectly with the #40. Especially if you have standard forearm, you can mount the 552 at the second or third rail slot behind the delta ring.

Yes, the #40, Colt front sight and the EOTech 552 will co-witness perfectly.

No, the #40 will not interfer with the sight picture of the 552 when it is in the fold down position.

Thanks  Just wanted to make sure they work work together...