Tamiya thinner can be used with MM and I think Gunze as well. Vallejo uses their brand thinner of simple distilled water.
I use plain old Krylon white primer, but you can use auto body primer (make sure it is the fine version, they make it in different 'grits') Tamiya and Model master also make their own primers too but at least the MM branded on I didin't like. (Plus they're more expensive than the off the shelf types)
Tamiya and MM can be mixed together, Vallejo doesn't like it in my experience, but Vallejo would mix well with artists acrylics I would think. The tamiya and MM have a different solvent than Vallejo, Vallejo is made with water versus a more chemical solvent.
I can't help you with the X-22 question... I do know that Swanny's site at: http://www.swannysmodels.com/TheCompleteFuture.html
about midway down is the mixing ratios for Tamiya flat base and future to obtain different levels of flatness. If the X-22 is the flat base it has to be mixed with paint or future, you can't just spray it on by itself. It would leave a whitish coat on the model and in the engraved detail.
Good luck to you!!