I used one on my 356 with good results. It was a pretty light pole and, as they all should, could be collapsed to swing through the fore triangle. Before I got the pole I used a boat pole or whatever. There isn't that much compression on it and I never bothered with a topping lift. What is does well is get the clew of the jib out where it should be.
Telescoping is good but, as was said you are limited by the J measurement. You do not need the heft of a spinnaker pole. There are spinnaker poles and there are whisker poles. They are different animals.
I know of no rule that prevents sailing with the whisker pole on the same side as the boom and I do this quite often while reaching. I've never been protested for that or heard of someone being protested. In PHRF you cannot have two headsails set at the same time except when changing sails and then only temporarily.
As for size on the 356, I think the tube size was around 2". Maybe 2 1/2". I don't remember.
So go ahead and get one, you're downwind sailing when sailing non-spin will be more fun.