The AIO has no 'standard serial cable on the side'. If it does then you use the Word clock module and only get WC I/O that way. The AIO will also not extend the Fireface because it runs on a different driver. In short: I am confused what you really want to do.

There was a breakout box, the RME Bob-1, including 2 m specially shielded D-subs. If you want to make your own cable just make sure that MIDI can not leak into the analog input lines. On the digital I/O make sure SPDIF coax quality is not reduced by too high capacitance or wrong impedance of the cable.

well, ok i just ordered the HDSPe AES ! still don't know what cables will be included. on the rme website it is mentionend that there's a 3m and 6m breakout cable available: BO25MXLR4M4F3 / BO25MXLR4M4F6. anyway, it seems i can't find them anywhere on the web. where can i get them?

on the other side i found this one: http://www.thomann.de/de/rme_bo25mxlr4m4fr.htm
will this be applicable for external gear via aes i/o to the hdspe aes ? or are there better/other alternatives ?

[my main task still is to overcome a 3m distance between computer and converters]