6 speed derailleur
 

Does anyone know if a Shimano 600 RD-6401 can be stopped down far enough to run a 6 speed uniglide
index mode or do I have to find a RD-6400 ?

TIA pwr

6-speed cassettes and freewheels have a wider spacing between cogs than 7-speed, so the answer is "no problem". Bolt it on, adjust it out, and go ride.

I'm using a 10 speed front derailuer on a 7 speed freewheel ,and it works fine.

For rear derailers, all Shimano stuff since the advent of indexing is theoretically compatible (except pre-9-speed Dura-Ace which had its own unique cable-pull ratio).
However, newer rear derailers designed for more gears have tighter tolerances. A 6-speed-era RD will shift very slugglishly on a 10-speed cassette, but a 10-speed-era RD will shift very quickly on a 6-speed freewheel.

To answer the OP's specific question, 6-speed freewheel is still narrower than 10-speed cassette, so the 10-speed-era RD will have no trouble covering that range.

For front derailers, there is the added factor that higher-speed chains are narrower, and a 10-speed-era FD may rub very easily on a wider 6-speed-era chain.

To give more information, I'm currently using a Shimano RD-6401 with a 6 speed freewheel with shimano 6-speed indexing dt shifters and it's working beautifully.

A caveat: I put a 9-speed Campagnolo Mirage rear derailleur on my wife's bike with a 5-speed freewheel and had to replace the inner (low gear) limit adjuster with a longer bolt, otherwise the derailleur couldn't be adjusted to keep the chain out of the spokes.