Think of the fish and filter media as paired. They should move together, within a few hours of each other. Move both across and wait a few days for the fish to settle. Then I would get a few more harlequins, test every day for ammonia and nitrite and if they stay at zero for a week, get a few more, and so on till you have all the harlequins you want. Then add cherry barbs the same way. Add the Dr Tim's when you get it, that should help (provided it hasn't got too hot or too cold on its journey to your house).
Once you have enough barbs, get the cories; by now there should be enough fish in the tank = enough bacteria to get all the cories at once.
Once you have the cories you'll need to add a meat (fish or shrimp) based sinking pellet to the food as they won't do well on flake. Cherry barbs are very good at picking over the floor of the tank looking for bits of food so they'll clear any left over flake. I used to have some
Leave the otos until the tank has been running long enough to grow some algae. Bns will eat other things besides algae so that could be added sooner.
It is common for otos to just die shortly after being added to a tank. There are two reasons - not enough food in the tank, and they way they are treated before you buy them. Always wait until they've been in the shop for at least 2 weeks so the weak ones will already have died, and choose the individuals with rounded bellies as that shows they are eating properly.