Hi Ben,
Mixed paint shouldn't be a problem, but I would suggest not thinning the mix until you intend to use it.
I would say leave it at least overnight, preferably 18-24 hours to allow time to cure. It should be fairly easy to scratch the paint off within say, the frst 6 hours or so after application.
Typically, when insufficiently cured, Tamiya paint will scratch relatively cleanly. If you scratch it after it's fully cured and the scratch penetrates the paint layer, the paint along the scratch literally shatters and under magnification, looks jagged around the edges, like shattered glass.
If you have any dried paint you wish to clean up, methylated spirits will do the job very nicely.