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External sample clock of USB 6009

Now I am trying to use two physical channels of USB 6009,  Dev1/ai0 and Dev1/ai1 to sample 16 slow signals. The 16 signals are divided to 2 groups, 8 in each group. I use multiplexer CD4051 (1 out of 8) to select the signal I want to sample.  The data select pins of CD4051 (A,B,C) is controlled by the Digital Output of USB 6009.
I used a 4ms timed loop to create the digital output signals on Dev1/port0. I measured the output and it worked well, ideally increasing from 0, 1, 2, 3,..... I used Dev1/Port0/Line0 (rising edge) as the external sample clock for Dev1/ai0 and Dev1/ai1. Dev1/Port0/Line1:3 were used to control CD4051's data selection port (A,B,C).
I used DAQmx Timing VI to specify the sample clock.
My problems are:
1. I cannot specify  Dev1/Port0/Line0 as the source terminal of the Sample Clock. The default terminal in the front panel is Dev1/PFI0. I connected Line0 to the PFI0 manually by using a wire. It still didn't work. The error says that the property is not supported.
2. I am not clear enough on how to set the sample rate in this situation. Because the sample rate is determined by the external clock.  In my case, the timed loop is 4ms so the sample rate could be about 250Hz. The figure may be not accurate enough and contain some errors.
3. When I set the sampling mode to Continuous samples considering that the whole sampling procedure is triggered by the outside clock (output of an inner counter), how can I split the data from Dev1/ai0 to 8 channel and regroup them.  Can I use Split Signals VI ?
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The USB 6009 does not support an external clock, thats the reason why you can not specifiy that in your program. You would have to use different hardware, that supports external clocks like a DAQPAd or an M-Series board.
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