Should I file these under additional income? I have about 4 companies I did work for in 2017 that paid me over $600 who haven't sent me 1099s.

Yes, you should report these amounts under Additional income in the self-employment section of TurboTax.

Thanks! And if I receive a 1099 in the future after I file it under additional income, is there any chance the IRS will get confused?

No. Just retain the forms 1099-MISC with your tax records. Businesses which have several forms 1099-MISC usually report a lump sum as Additional income and do not enter the 1099-MISC individually.