Make sure the shark has a little cave or hiding spot. That will keep him happy and less aggressive. Do not put any other long bodied fish in as they tend to fight with them (SAE
, Fly Fox etc...). The Khulie loaches do well (as you say) in groups of at least 5. The sand will be great for them as they like to bury themselves and dig around. Perhaps a layer of laterite or flourite put in before the sand will help with the plant growth. Ram's would work great.
A scool of smaller fish would be good. Tetras come to mind: Rummy Nose, Neon, Cardinal, Glowlight, Black Neon, Silver Tip. Or Rasboras. Or if you like barbs then cherry barbs make a great addition and if kept in groups of around 5 or so then they keep any aggression in the group. Rosey barbs area bit bigger but they are well known for their ability to eat algae off of plants so they would be a great choice.