Barry, you can use most types of screws in aquariums but do not use galvanised screws because they can leach zinc into the water and kill fish. Copper screws can kill shrimp if there are lots of screws but a couple are unlikely to cause any problems.
An alternative is plastic cable ties. They come in a variety of lengths and can be used to hold bunches of twigs together. You can also use super glue, as mentioned by others.
An alternative is plastic cable ties. They come in a variety of lengths and can be used to hold bunches of twigs together. You can also use super glue, as mentioned by others.