backup and restore questions

I had a few questions concerning the backup/restore process for pg.
 
1) Is it possible to dump data onto an existing database that contains data (assumning the schema of both are the
same).Has anyone done this? I am thinking of this in order to expediate the data load process 
 
2) I read that when dumping and restoring data the insert option is safer but slower than copy? Does anyone know from
experiencehow much slower (especially for a database containing millions of records). 
 
3) can pg_restore accept a file that is not archived like a zipped file or plain text file (file.gz or file)
 
4) Is the general practise to have one whole dump of a database or several separate dumps (by table etc...)?
 
Thanks a lot
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