Hi,
Your 240 litre tank holds 52.793 imperial gallons or 63.420 US gallons. All the fish you have mentioned can go by the rough guide of one inch of adult fish per gallon so you have quite a lot of room to play with, though it is always better to under stock. If you add 2 male guppies to the females, each female will have babies every month so unless you really like guppies it would be best to get all of the same sex. Females will often come from the shop pregnant and can store sperm for around 6 months.
8 female guppies (may add 2 males instead of 2 females) 2 inches so 16 inches of adult fish
6 corydoras ( not too sure which variety)average of 2.5 inches so 15 inches of adult fish
6 black widows 2 inches so 12 inches of adult fish
8 neon tetra's 1.5 inches so 12 inches of adult fish
1 bristlenose plec depending which type 4.5 to 6 inches on average.
So if you add up all of the fish you are under 63 inches of fish.
The only thing to watch out for is the tetras nipping the guppies, especially if you get the males with long flowing tails. Personally i would swap the guppies for some marbled hatchet fish which grow 1.5 inches. You could also add some fan shrimp or ghost shrimp to this set up.
Fishless cycling is the way to go so read up on all the pinned topics.
Hope this helps
Emma