Try Diafine. You can develop all films and plates for the same times (directions on the box). Diafine is a 2 bath developer. I think you develop for either 3 minutes or 3.5 minutes in each bath. As for the plate speeds, try shooting one plate at ASA=10 and one plate at ASA=100. Those will usually get you a starting point. The slowest glass plates I ever tried were Kodak Metalographic plates, which I found had a speed of about ASA=0.01. Diafine will usually give you good pictorial quality negatives even from high contrast scientific plates.