I tested FreeMiCal output with Google Calendar.
It works fine.
However, there is a quota limit on the number of calendar items one can add per day. I haven't found out the actual number, have to test a little.
Also, if you have more items in the export file, the server might run into a timeout. It leaves the import in an unfinished and undetermined state.
So far I am confident, that 500 items are OK.