the market prawn is to speed up the cycling process..but u have to monitor the water parameters closely..
its not a must but most people does it to speed up the process of the breaking down of nitrates..
the small mushroom will survive depending on your parameters and lightings..
does the mushroom appears to be fully open or shrivelled up?
you can add "stockings,socks,coconut bag" at the outlet of your skimmer to help to reduce the number of microbubbles
just my limited knowledge