I'm a sole proprietor and recently got an EIN because I subcontracted a few folks. When filing est taxes now, do I use my EIN or SSN? Are they interchangeable?

you'll find that when using IRS Direct Pay or for your  estimated tax vouchers all you can use is your SSN.   the EIN, mostly for identity theft (no employees) can  be used to for 1099-misc   and is required if you have employees and are making payroll tax deposits