I have connected the fibre, but the receiver module still has a red LED on

It is very easy for a fibre end to be contaminated with dirt. All PPM equipment is supplied with basic cleaning kits. It is always recommended that a fibre end is cleaned before inserting into the connector. Also, FC/APC connectors have a single slot 'key' position that must be mated first before the thread is tightened.

What type of fiber optic connectors can I use with PPM's systems?

Optical connectors are critical to the performance of a fiber optic link. All PPM ViaLite products use FC/APC connectors to minimise back reflection of light into the laser. PPM recommend the use of Huber and Suhner part number FCPC-Z/M-A601 which are FC/APC narrow key connectors offering >60dB return loss. PPM also offers the E2000/APC connector standard.

Is it possible to carry multiple carriers having quite different powers beyond a module signal range? 

The out-of-band performance of the fibre optic links is not specified.  Typically, a PPM L-Band module will have some response from, say, 400MHz to 2.5GHz and no filtering is included to limit this. It is suggested that if you have out of band signals which might compromise the IP3 of your link a filter / diplexer should be used in combination with the fiber optic link.