Hi WW2Spirit and welcome to the forum.
This question has been banded around in one format or another on quite a few occasions. I think you will find that it is a matter of perspective. My view is that generally, the scales are a little too different to mix up, especially if you're planning to use them on the same diorama.
You might well get away with some of the vehicles, but I think you'll find a noticable difference with the figures.
It gets more complicated than that as well.
Take the 'middle' scale of 1/32 if you put a Conte figure next to an Airfix figure next to a Gun Ho figure, they will all appear to be of a slightly differing scale, although all are marketed as 1/32.
That said, if you take in to account the varying height and build of a person, you could quite conceivably make this work.
If you gave us an idea as to what figures or vehicles you had in mind, I think that there will be plenty of responses as to whether they would work together.
Of course, don't expect the responses to be the same
but it would give you an idea before reaching for your wallet.
Simon