I have used several different biocides for diesel, and I think the major ones all work. You might want to consider a small fuel pump and a filter set to continuously, or more reasonably periodically, filter your fuel. If you make your prefilter one that is a water block, you can help get water out of the fuel, and then run it over a fine filter. (Put the water block filter before the pump to keep the pump from emulsifying the water.) Your engine will appreciate it.
Personally, I like to keep my diesel tanks full to limit condensate issues, but YMMV.
If you plan to use your generator in the winter, I would recommend a block heater, oil heater, or an air intake heater. Any one of them reduce the startup load, and increase the reliability of your setup, which is the point, right? The first two will help keep things warm, which also reduces condensate issues...
One thing not to do is use biodiesel in a tank that isn't going to be used for awhile, e.g. a standby generator. Algae and bacteria grow way too easily in biodiesel.
That is a beautiful looking shed.
All the best,
Peter