Allen, your cracks are, in my opinion, too large for fillers. The filler will crack and not stay. The wood will still move around and fillers are not flexible at all.
If you go the filler route anyway, be very sure that the filler is stainable. So much out there is not stainable and looks bad when you start the staining process.
Is there anyway you can correct the cracks with tightening up of the existing planks.
Another area that some others will disagree, but when you dilute or thin or add to make vescosity less, you are going against the molecular structure of the product. So, I would not thin the product even though many others will disagree.
I would though do this on a warm day or have the room at a normal temp. It does act like molasses when real cold. Are you using oil or water poly?
Have a very good dust management program in place when you begin the poly job. dust is the main foreign particle that makes a oil floor job a wreck. Blow, suck, wet mop, install curtains, change the HVAC filter, shut off the vent to that room, close all the windows and doors, (but have some ventillation so you don't croak). You get the point.