As Ray said, it is certainly OK to mix gut and nylon if you choose the correct gut sizes. Another option would be fluorocarbon which has somewhat higher tension than gut for the same diameters - Savarez Alliance, for instance. I rather doubt that either would work very well on the lowest strings of your 34, though. One possibility down there would be fiber core, silver or bronze wrap which are available from Vermont Strings. Now we're getting into expense! But it may be worth it if you really are having trouble with the lowest ones and want the smoothest transition from nylon to gut or FC:-) Personally I would not put those fiber core on the Ravenna but an FH - perhaps. The standard bronze core nylon wrap however already sound great to my ear.