When dealing with fish, a bigger system with more water is usually more stable.

I like this idea in concept. However in most cases failure is mostly due to power loss. So unless these seprate systems have different power sources it might not protect from catastrophy that much.

As some one who does aquaponics at home. Here are some things ive learned which you might consider.

• There is a minimum physical footprint that these systems use, especially if you are trying to conserve power. So it might make more sense to max out those systems in that minimum footprint.
• Different fish can have very different temperature and pH requirements.
• The pH and temp that the fish like isn't always what a plant wants.
• some fish have much greater ranges of pH and temp requirements than others.
• if indoors, it's easy for a pest to establish. Like aphids will trive without ladybugs or green lacewings.
• Vegetables grown in aquaponic systems simply do not taste as good as vegetables grown in soil. Often they are lacking flavor.
• something about the water in an aquaponic system is excellent for propagation from plant cuttings.

For these reasons, I use aquaponics more as a backup and complimentary grow system to my no till regenative garden. Having an indoor system is great in the winter if you want leafy greens or to proprpgate trees for the next season.

Decentralized agriculture

I would make it big and use just one crop and one kind of fish, and then crossbreed the crop with the fish so they share genes but have better yield and can be eliminated by a single virus popping up and killing it all

I have to say, I'm always skeptical of proposals that seem too active.

Agriculture is mostly passive. Of course plants need care, but most of the time, they're doing their thing alone. If you need constant monitoring, maintenance, resource cycling, etc, etc, it'll drive costs up very quickly and will need to be detached from nature quite a bit.

This is, at best, a niche solution for rich countries. It won't feed the masses.