Maintainability: is declaring serials as integers a problem?

I have tables that have seirals as primary keys. Other tables uses these
as foreign keys. In terms of future maintainability is it best to
declare the foreign key as:

1- id integer references other_table

or

2- id serial references other_table

I'm a bit bit confused is a foreign key can (should) be declared as type
integer or serial.

Thanks!

jc