I'm wondering if the SD Output from the F900 (through the 3:2 pull-down board) would be letterboxed? I'm asking because I'm hoping to view the output from the 900 on a SD monitor but the monitor's aspect ratio is 4:3 not 16:9. Meaning I'll get a stretched image, and that's no good for monitoring.

Gday Andrew, the F900 classic does not have composite out, only component, which will put out a squeeze on a 4:3 monitor. The work around that we had to deal with was to put a miranda down converter on the back of the V-mount, which will not only put out a composite signal but also HD-SDI.
If you have the F900r then that is different because that has composite and HD-SDI as standard and also a D-tap hidden just under the top handle for on board monitors. I hoped that helped.

It's 16:9, so you'll need a SD monitor that can de-squeeze into a letter box. You can get older monitors modified so they're switchable between 4:3 and 16:9.

the 750 which is the build and menu that 900r is based on allows you to choose how you want to see it (letterboxed anamorphic or edgecrop) the hdx 900 has the same choices. but the standard f900 does not have sd out actually the 3:2 pull down board is only applicable to the hdw-f500 which is a deck, and will allow you to see sd in whatever form you want.

We're looking at using the three 5.6" monitor from sony.

http://bssc.sel.sony.com/BroadcastandBusiness/DisplayModel?m=10007&p=8&sp=20131&id=77623

So, what would be the best way to get a 16:9 image to that monitor from a F900R, F950, and F23. The PDF file says it can only do 4:3.

We're looking at using the three 5.6" monitor from sony.

http://bssc.sel.sony.com/BroadcastandBusiness/DisplayModel?m=10007&p=8&sp=20131&id=77623

So, what would be the best way to get a 16:9 image to that monitor from a F900R, F950, and F23. The PDF file says it can only do 4:3.

I'm probably a couple months too late, and I have not worked with the monitors you listed. But I thought it should be noted that the SD output on the F900R is only available if the body has the optional HKDW-902R installed. When you select the VBS SD output in the menu, you can choose either SQUEEZE or LETTERBOX. So regardless of the monitor's capabilities, you should be able to get the aspect ratio correct.