Pretty sure a 401K has to be set up and funded before year end.  The "company side" of the contribution you might be able to fund up until the filing deadline but the plan must be set up still by year end.  A SEP IRA can be set up until the filing deadline.

I think you can have both but you still have to follow the contribution limits.  Meaning, if your allowed SEP contribution is 25,000 and the solo 401k has a limit of 50,000, the total limit is 50,000 but you can put 25,000 into the SEP.  And if you did you could only put 25,000 into the 401K,  Of course, within the 401K you also have the employee & employer limits you have to monitor.  It's sort of complicated to have both so you have to be careful.