Responses to your questions:
1. Yes, that is just a starting point
2. Yes, you can increase the SDE tablespace as needed. I would recommend auto growth with an unlimited size. Something like this:
ALTER DATABASE DATAFILE 'DATAFILE_LOCATION\SDE' AUTOEXTEND ON MAXSIZE UNLIMITED
There should be no impact to the function of the GDB based on the size of the SDE tablespace. I would think that you would have more issues if you ran out of tablespace room than have too much.
--- George T.