Chris Parker
Grandmaster
For a final point, I would recommend looking to the quote from Kyoshi Troy Wideman in Bruno's signature. That says it better than most.
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You are forgiven...
No, it is not a case of "just using whatever techniques work in a situation". It is a matter of adapting the principles of the art to a situation. And the principles of the art, it's guiding philosophy, will proclude you from a number of techniques, for example roundhouse kicks and axe kicks go against the principles of Ninjutsu. However, they work perfectly for Tae kwon Do. In that art, though, Ninjutsu's postures are all wrong.
The point here is that the underlying principles, or the guiding philosophy of an art, give it it tactics, strategies, power source, and more. You just can't use an Aikido power source and expect it to be effective with a Tae Kwon Do attack, and vice versa. I once had a "discussion" with a person (who trained in a bogus ninjutsu school here in Melbourne) who was telling me that roundhouse kicks and three-sectional staves were perfectly fine to use in ninjutsu because ninjutsu is whatever a ninja uses... kinda missed the point on that one. A ninja is not just a guy in black pajamas and a hood, you know...
And Hudson69, provided you could congruently combine the different techniques under one principle, then yes, you could get benefits out of cross training the way you say. However, most people cross train as they believe they have a lack in one or more areas in their current training, so they go to a very different type of school to cover that gap. And that honestly just doesn't work. MMA training, for example, I have said a number of times is not really "Mixed" Martial Art training. It is a single system with a single guiding philosophy which works with multiple ranges, which grew out of disparate arts.
If you have managed (as MMA coaches have) to bring everything together congruently, then it can work. But you will find that you are not really using multiple arts, when it comes down to it, you are using a base system (probably Kempo for yourself, although not necessarily) as your guide, and simply adapting the other techniques to that first arts concepts. So bringing in ideas from other arts is not in itself bad, provided you can make it work in your original art. As said, a spinning tornado kick won't work within Ninjutsu, but groundwork (adapted from BJJ or similar) can.
The reason I say you will adapt it to a single art which may or may not be Kempo for you, is that the art you will naturally go to in a high stress or high adrenalin situation will be the one you unconsciously believe is the strongest, and that may or may not be the one you have spent the most time training in. It could be the one that most closely resembles what you see as powerful (say, from movies), or could be an art you haven't studied for years or more, but you had a very powerful experience in which has never left you. Hope this has made sense...