Mark,
Yes, you should be able to use PFI0. To do this, add the DAQmx Channel property node between create channel and start on the counter task. From this node, select the Counter Input->Count Edges->Input Terminal property. Set this to PFI 0 for your device.
For error -200673, it sounds to me like somehow the waveforms you are writing to your analog output channels contain more than a single sample (ie, the waveform's Y component is an array with more than one data point). I would validate two things:
1) The output of the zero value waveform I construct for the case where the counter's count > 0. I've mucked around with the VI enough that it's possible I may have messed this up at some point.
2) That the waveforms coming from your feedback subVI are valid. As a debugging step, I would add code to your feedback generation to throw an error if any of the waveforms it generates have a Y component with a size greater than 1. If you've been using the counter, I would also validate that the the 'zero' output waveforms I construct are correct.
Hope that helps,
Dan
