Hm. That's a tough one, because there are a number of arts and systems that can all be said to be "internal," but they don't all share the exact same "package" of internal qualities. Some may be missing an element or two that another system has. And, even if there is some overlap, or two arts may share all of the same principles and concepts, they may express and utilize them in very different ways -- which is what makes different arts look and feel different from each other (e.g. bagua, taichi, xingyi, etc.)
That said, if they are all indeed using the same basic method, IMO you can integrate and incorporate application ideas from each. A lot of taichi masters sent their students to learn from a bagua teacher, a xingyi teacher, and vice-versa. In the Japanese arts, the same thing happened. There were Daito ryu students who snuck off to train with Wang Shuchin, and sword students who went to study Daito ryu, in hopes of gaining the power and efficacy of internal methods that they could integrate and apply within their own "home" arts.
Some of the best arts today are amalgams of several different arts or systems that have an underlying power-structure-body method that is the same or close to it. The shared physical principles and qualities of body movement were compatible; hence, the arts could be merged and clarified into something which was new, and yet drew its substance and strengths from some very ancient and well-tried sources.