Look into using an 8051 with the the PCA. This has 5 channels of counter capture/compare logic built on board. In Capture mode there are 5 inputs that are the capture controls for each of the capture modules. You can use these to detect the high and low pulse widths of the RC5 inputs. And the inputs can be set to alternately detect the going low and going high edges of the input signal.

It should be relatively easy to thus use one microcontroller to monitor and decode up to five IR detectors. This seems to me like a good amount. I say that because then you can arrange to put an 8051 near to a group of 5 detectors and keep the wiring to the detectors reasonably short. Then you can use a protocol like RS422 or RS485 to bring detected IR information together into a centeral control processor.

This is important becasue the output of a typical IR detector cannot drive a long cable. (Although I have worked on a system where we put a RS422 driver right on the output of an IR detector and fixed it up so the combined module could easily drive a cable of up to 250 meters back to the location where the IR was decoded).

Good Luck.
Sounds like a good project.
Mike Karas