I don’t see how you can fry a fooger by going wild with CVs between foogers. The CVs on foogers are designed to work within a certain range. All the CV outputs will be in that range so you can really fry anything by plugging fooger into fooger.
If you use an external voltage source, I would just make sure its within or close to the foogers range. For example, don’t plug AC current from your wall into the CV. That would fry the fooger and probably make it catch on fire. But as for fooger to fooger madness, go crazy.
I guess it would be like connecting a battery or transfomer to the same. It may create some sort of short which in itself would probably not destroy the fooger but may create heat enough to harm the components. Some of the foogers like the freqbox have ICs.
Do we have any electronics experts here? I wish I was but that is not be. I understand the basis of circuts but if this would cause harm I don’t know. I could be good to know. Foogers are not cheap.
An out to an out can definitely cause harm if done regularly. In’s should go to outs ALWAYS to achieve new sounds. An out to an out won’t make new sounds, I don’t think.
The other bad thing would be to combine two outputs into one input without attenuating. Some of the CV ins can be damaged if you put more than 5V into them. But if you always combine CVs using something like the CP-251 mixer, it will be OK.