With all the new Democratic pollies can they over-rule a presidential veto?
It's possible but unlikely, especially in this era of characteristic non-cooperation between the parties. Around 4% of presidential vetoes have been overridden historically, and certain controversial figures (like Andrew Johnson, whose vetoes were overruled 15 times) drove that average up more than others.
We'll know more when all seats have been assigned, but it is not expected that the Democrats will have 2/3 majority in either chamber, so Republican support would be needed. There is only one national issue on which I could imagine such a vote succeeding: Iran.