More important than "formed" or "founded" in this specific example is this:
"The supermarket where I work opened in May, 1999."
When or how it was formed or founded is generally irrelevant, unless you are legally referring to when the organization that owns and operates the supermarket was incorporated, in which case that is the correct term to use.
In the non-profit sector, I would say that "founded" is generally preferred over "formed," in my experience. One might say, for example, that "The Vector Health Centers were founded by four physicians in 1976 and incorporated as a non-profit organization in Delaware in 1978. Vector is supported by numerous foundations, private gifts and public grants and cooperative agreements. It currently operates 23 primary care medical clinics in rural communities in five states as well as three countries in sub-Saharan Africa" (this is a wholly fictional example).
I have to agree with commenter Hot Licks that "formed" is vague and less specific than "founded" or "established" (as suggested by jamesh, above) with regards to the OP's questions.