Will, it's difficult to be 100% sure if particular mixes are compatible or not without a test.
What I can tell you is that I swear by Mr. Leveling Thinner
I have mixed it with all the paints I have in my "library" - except acrylics, obviously - and it has worked out fine every time (Zero, Gunze Mr. Color, Tamiya TS paints).
I also have Mr Retarder but don't actually use it as I have never found the need to additionally "slow down" the paint but maybe in your own climate it might be useful?
HTH