Previous and former are both correct; however, former has a negative connotation, so you'd want to use previous.

Both are correct~although if you say, "the company for which I used to work..." people also understand (because of "used to")

The old company I used to work at = my previous/former employer In your example, only "previous" is correct.

I guess probably you can use "previous employer". Also looking forward to answers from native speakers.