As for the Smallest requiem sharks - are you wanting to know the small species of Requiem sharks or the Smallest species that is currently available to private aquarists?
Also the most suitable species of requiems - generally aren't the smallest species.
The Smallest species of Requiem Sharks - is generally about 30" or less -usually such species as the Brazilian Sharpnose & Australian Sharpnose. But these species aren't available to aquarists.
The Smallest species of Requiem Shsrks that is available to aquarists - is the Atlantic Sharpnose at about 3.5-4 ft long.
The most suitable - in terms of hardiness & care would likely be either the Blacktip Reef or the Whitetip Reef.