Hi Nichole, let me know if you'd be interested in the PDF referenced in the second link that Kim posted above. Unfortunately, Marketo won't let me post it here. My email is dviscogliosi@emarketer.com
And to answer your question, Marketo won't mark 1000 soft bounces as invalid. You'd have to set up a rule on your own that would mark email invalid after x number of bounces.