Are you rigging them all on the same mast?
The 9.5 would seem to want a very stiff 490 or 520 mast. (IMCS 29-32)
The 7,8 m2 would probably use a 460 IMCS 25 mast.
The 6.8 m2 would for sure need a 460 cm IMCS 25 mast.
If you rig on the correct mast, the downhaul tension should not be enough to damage your downhaul
If you are trying to "bend" masts that are too stiff into your smaller rigs that need softer (more bendy) masts, then you can probably expect the downhaul tension to be sufficient to damage the downhaul line.
Also, look at your downhaul crank tool...... are there nice radiused edges on the hole through the crank shaft?
or fairly abrupt and sharp countersink edges that can cut/ damage your downhaul line.
Are you using one of the North Mast Systems.... with different tops and tapers?