newer tank clean up crew

so from what i understand there specific recommendations of how many crabs and snails i have per gallon. but here what im concerned about: first off cost, i got a 72 gallon tank and my local shop sells snails and crabs for $2.99 a piece. i seen some that were way more affordable online, but Im unsure about the quality. second, beings that it is a fairly new tank with minimal live stock (two clown, one tang, one royal gramma, three peppermint shrimp, one cleaner shrimp, three hermit crabs, on emerald crab, twonassarius snails, and one turbo snail. and some candy cane corral coming back to life from the previous owner.) do i need to ad all that stuff right away? or should i do it over time. and if i do add it all at one time will there be enough for them to eat, and where should i buy them from?

thank you for you help. :eek::help: