One of the problems in mixing inks of any brand is the unknown composition. You really cannot predict the effects which could take some time to manifest. Since you do know know what chemicals you are mixing, you can't state with any confidence what will happen, even if that is "no effect".
For pens with converters the possible problems are few. So mixing may be quite possible with care.
However for pens with sacs and diaphragms the dangers are much higher due to possible interactions with the latex or rubber. I present this from first hand experience with a year-old Vacumatic-style pen with a diaphragm. The pen was having problems filling. The manufacturer took it back for repair and the diaphragm was failing. The only cause could be the ink. Sailor and OS were the predominant inks used with one fill each of Eclat de Saphir and Asa-gao. All of these inks are pretty safe afaik and yet somehow in combination they may well have caused a problem.