If your classes are the same level, they will have the same skills. If their gear is the same strength, they will get the same progress and quality from synthesis and touch actions. If you can do something on one class, you can typically do it on another. The exception being classes you have the specialist stone for. Those classes get a small stat boost(20 craft, 20 control, 15 cp), and have one extra action they can use(careful observation) - also a very small number of recipes are locked to specialists only.