i tested it for a while and it seems that they are not obtainable this way. Is there any chance that new fishes will be obtainable this way?

The new fish we added should currently be obtainable from fishing but are only catchable in specific biomes.
Make sure you are testing these conditions:

Or i don't see a way to disable fisherman trades in the config, i think this should be a thing

Is there any way to change it by myself in the mod to have them in all biomes?

It is not currently possible to customize where the specific fish from this mod can get caught.
We are planning to make all the loot table modifications use JSON files to make them customizable.
We will consider adding a config option for the fisherman trades.