How many gold fish would you fit in a standard bath?
Depends on whether you aerate and filter it. I wouldn't put more than 4. You can have more when they are small but they grow fast and poo lots.
Do you need to keep the bath out of the sun?
Preferably have it shaded part of the day or it will go green very quickly and you won't be able to see the fish.
Do you need to have any water circulation or airation?
No
Do you need to feed much or do they mostly live off insects?
No you don't nbeed to feed much. Yes they will eat insects falling in.
Can you buy plants from garden centers or should I go trade me?
Do both or gather oxygen weed from your nearest river
Which goldfish varietes can live outside? specificaly shubunkins.
As far as I know, "shubunkin" refers to the colouring. You can have different types of shubunkins.
Comets are good outside and fantails and veiltails. Depends on where you live and how cold it gets in winter.
Can gold fish live with WCMM or tadpoles?
Yes, they will enjoy eating them very much
Do you need a substrate?
No