you can reduce the size of a datafile, providing there is NO data in the space you're removing (in the last 495 Mb in your example).
what you should do is :

drop table huge1;
drop table huge2;
...

once it's ok :

alter tablespace [tbs] coalesce;

and then :

alter database datafile 'your datafile' resize 5M;

if this command fails, then you certainly have data in the bad area, you should look for what it is by looking in dba_extents and dba_data_files ...