Plain 5 - 10% gelatin is just fine, but I suggest adding some liquid (not gel) food dyes to it so that you can see the quality of the coating clearly. Clear gelatin is very hard to see when dry. A hardener is not needed, but a surfactant might be useful. I would suggest about 1/2 ml of Photo Flo 200 to every 100 - 200 ml of 8% gelatin as a test. Do it without and with and then compare the two. If the coating is worse with the PF200, then there is probably too much or too little as the response to surfactant is a curve with the optimum in the middle and worse positions to either side of it. Kind of a bell shaped curve.

I coat at 5 - 10 mils and use about 12 ml per square foot at 5 mils gap.

You can coat film at 100F with cold or hot film, but paper should be coated cold with hot gelatin. Heat changes viscosity and uptake into the support (if any).

This is a good method to get started.

PE