I am going against your premise and suggest you will get a better feel for your film and developer if you do stick with one developer but work with 2 different films.
Get yourself a slow film and a fast film and use them both. Stick with just those two films and stay with just the one developer. I would suggest for films Tri-X since you seem to like that idea and FP4. Working with 2 films you will be able to see differences between the two which will tell you more of the characteristics of each.
I actually didn't come up with this idea on my own. I went to photography school and that is what they taught us. We started with Tri-x for fast and Pan-X for slow and processed both in D-76 and worked that way for a year. Without experience and starting with just one film is like looking in 2 dimensions. Starting with 2 films and using them together gives you a third dimension and will allow you to notice some individual characteristics.

Dennis