If you are using tables, just use the same tables for DC as for AC. In other words, you have 6 CCCs and must derate your conductor ampacity by 80%
If you are using 'engineering supervision' then the results will be slightly different DC versus AC, because of the various loss terms associated with the AC voltage applied to the insulation.
However I suspect that in most DC systems once you increase the size of the conductors because of voltage drop requirements, you will find that derating of ampacity is not an issue. In the example you gave, you have 500kcmil conductors because of voltage drop. The normal 60C ampacity of these conductors is 320A. With 6CCCs, the ampacity is 'down' to 256A, well in excess of the 100A that you are using them at.
The thermal effects of having multiple conductors in one conduit may be an issue for you only in that the resistance of copper increases as temperature increases. You should keep this in mind when designing your circuits to meet voltage drop criteria; don't calculate the voltage drop using the 75C resistance; use whatever temperature you expect the wire to actually be at. This temperature will be above ambient; the wire self heats because of current flow, and the ampacity is set by the current which will cause the wire to self heat to its temperature limit. Multiple CCCs in one conduit mean more heating in one place, and thus higher temperature and higher resistance.
-Jon
