AFAIK, there are 2 limits:
Interesting to know these limits.. I knew there was a (somewhat) arbitrary number around 2100, but that makes sense that there's also a character limit -- and also why the limit never seems to stay the same
I Just do not remember where I read this, so may be I am wrong, but yes, it would make sense.
In this circumstance, I would import to a static list and tell Marketo to skip new leads. That way, only the records that are in your system get added to the list. Not sure if that would work in this scenario - guess it would depend on your goal