It is my impression that the developer isn't sensitive to temperature changes within a certain range. I've used the developer at 80*F and at 65*F, without noticable difference.
In regards to your film choice - I have no clue. I use it with Tri-X which I rate at a thousand, but TX seems to be the one film that benefits the most from Diafine in terms of speed. I would probably start at box speed and bracket one stop to 800 on the same roll if you can. (but I favor fairly dense negs).
- Thom