[JPPLAY]

You already got my opinion on gun sizes.
The Omer Cayman is too expensive for a 75 cm. Make sure you get the lower version of the Caymen not the HF. The special design of it to be stiffer is not needed for what you would be doing with it. Get something cheaper and in aluminum so you don't have to worry about scratching it. The standard Caymen or if you can find a T-20.
I shoot a Beuchat 90 cm but I got a good deal on it and until you get picky about what you shoot then you can spend the extra money. I have also learned how to handle the gun in low visibility situations but a 75 cm would be handy.
Good spearfisherman can win in competitions with basically any brand of speargun. It comes down to your skills. As you get better you will be able to get away with shorter guns as you will be able to get closer to the fish.
I can't comment on the fish you are hunting as I haven't hunted any of them unless by panfish you mean perch. Just go out and dive. Don't be afraid to try new things all the time and when the day is good to try other techniques to see if they work as good or better. Just because something didn't work once doesn't mean it won't work latter.