I don't know about the developer. X-ray developers are fast working and very active; probably closer in composition to a paper developer than a developer designed for general photographic use. Fixer is fixer. It is either ammonium or sodium thiosulfate, plus some addenda to preserve and sometimes acidify the solution. X-ray fixer is likely nothing more than your standard rapid fixer using ammonium thiosulfate as the fixing agent. Use as directed or dilute it to a strength that suits your purposes. Personally, I use Kodak's C-41 Flexicolor Fixer and Replensiher for all my B&W fixing applications. It's an off-label use, but it works and works well. It's a bit strong for B&W work when used as intended, but it is very fast. I dilute it to half strength, and it still works fine. Approach using x-ray fixer with the same methodology. You'll probably be pleasantly surprised.
Timing? Who knows? Once your film is in the fixer for 30 seconds, it's ok to turn on the lights or open the tank. I do it all the time. Keep track of the time it takes to clear all the milkiness and double that for your fixing time. Papers fix out more quickly than films. Figure half the time at most.