The range for 'descent rate' is 300 to 6000 fpm. It determines the maximum rate the plane type can attain during a descent (although it will descend faster when 'expediting'). So a setting of 300 to 500 fpm should work fine; lower values will be constrained to 300.
The 'descentaltitude' is indeed the same for all plane types. The idea is that it avoids conflicts with lower planes, keeping the gameplay more simple. For the default airports, the descentaltitude is set below 10.000 so it's not really an issue there. To work around this game limitation, you could perhaps keep the descentaltitude below 10.000, but also add a (higher) custom altitude to the 'entrypoints'. Light aircraft will still be constained by the descentaltitude, while other aircraft enter higher.
I hope this helps, thanks for the feedback!