40 cm might be pushing it for an adult but will be fine for a juvi. adult angels get very tall and require room to move about the tank up and down as well as side to side without being cramped.
adult angels can get to 5-6 inches nose to mouth and 6-7 inches top to bottom depending on their environment and conditions etc hence the reason to need a taller tank.
for dithers pretty much anything thats bigger than a neon tetra should be ok. basically anything that will fit into the angels mouth will become a snack. ive kept cardinal tetras with no issues alongside angels but anything smaller would be a risk tbh. it might be safer to go for a fuller bodied dithers if you can only find smaller specimins at lfs.
also when buying dithers i would go for as bigger group the same species as possible. this will make the dithers feel more secure in numbers of their own and will look much more natural and have a greater impact on the tank allowing the dithers to act more naturaly. its very tempting to get say 3x fish from 3 different species to add more colour and variety but 9x same species fish will be much more suitable and look much more impressive too