At first thought 6 months seems not that long, equivalent to a standard probationary period in many jobs. However, it's probably not as straightforward as that when training and associated assessment is taken into account.
Is there a distribution of crew experience in each cabin, so less experienced crew are working with those who have more? Or is it possible that the situation may occur that a cabin could be crewed entirely by relatively inexperienced crew with only a year or so's worth of experience under their belts? I'm not saying this results in a lower standard of service, just musing as to how it works.
From what I understand, Mixed Fleet crew request where they would like to work but the CSM makes the actual decision and would be expected to try and get a good spread of experience across the cabins and doors for safety purposes.
When rostering cabin crew, the computer treats anyone who has done more than six months as experienced, as this is the legal stipulation for CAA purposes, so although a CSM will try and have a good spread of experience (and of course at least one or two on each flight will be FTCSMs, so will definitely be experienced) sometimes it is hard for the CSMs, as sometimes the system gives them a fairly inexperienced crew to work with.
I am sure whenever possible they will do as you say and make sure that one of the crew in First is experienced enough to guide the others in the cabin and of course on many flights all will know what they are doing I would hope.