Shut the cats in another room whenever the hamster is out of his cage. Get something to secure the screen top (clamps or velcro strips or whatever).
There's no way to expect the cats to ignore the hamster, because small rodents are their natural prey and their instincts will take over. So all you can do is make sure the hamster is safe.
No, more likely because being in a bowl is fatal to goldfish. They need a lot of room and very powerful filtration. Pet stores are terrible about selling inadequate enclosures for goldfish.The fish died naturally, could have been cat caused strokes?