how crazy is a wine fermentation? Could I put 6 G of wine is a 6.5G carboy and be ok?
Todd the correct answer (imho) is 'IT DEPENDS'. I use 12 US gallon primaries to start 6 gallon kits and have had ferments touch the lid. Rare, but it happens.
Personally I would not use a carboy as a primary for three reasons...
1) size
2) difficult to add ingredients to carboy
3) difficult to stir well enough
Steve