Mixing might work. Check Sheldon Brown's site on cog spacing, and you'll see that 7 speed HG Shimano and Sun Tour Ultra 7 have the same spacing at 1.85 mm. So if you use Shimano shifters and freewheel/freehub, along with the Sun Tour RD, things might just work. An 8 speed cluster might work too, since the spacing is 1.8 mm, and there may be enough slop in the system to work.