Can et al. be applied to companies?

The following abbreviations might serve you well if you want to sound fancy:

et al. (et alii) and others
etc. (et cetera) and the remaining ones
inter alia amongst others

caveat scriptor: if you use latin abbreviations gratuitously you will want to make sure that you know more latin than your coworkers!

Et al. is an abbreviation of Latin et alii (masculine), et aliae (feminine) or et alia (neuter), and it means ‘and others’. As the abbreviation doesn’t indicate the gender, it can be used to refer to men and women, and to inanimate entities, leaving the reader, if sufficiently erudite, to supply the appropriate ending.

However, a writer will pay greater respect to the reader by listing the others, rather than hoping the reader will be able to guess what they might be. If they really are too numerous to mention, then English rather than Latin will serve perfectly well, and sound less pompous, with something like ‘and similar companies’.

Yes. A company is a "corporate body" (body corporate).

Legal personality (also artificial personality, juridical personality, legal entity and juristic personality) is the characteristic of a non-living entity regarded by law to have the status of personhood. [emphasis mine]
_(Wikipedia)

Sure, you can.

No, you shouldn’t. You should not use a Latin abbreviation. Just use English.