Recently I developed Tmax400, FP4 and HP5 together, in Pyrocat HD, with no problem. My times for these films would only vary very slightly if I process separately.

However I made a huge mistake processing my first rolls of Fomapan200 alongside Tmax400, as I've since done some Zone system tests and discovered that my dev time for Fomapan 200 should be 10 minutes and not the 15 minutes I gave them. 15 mins was equivalent to N+2 so the negatives were excessively contrasty.