I am not sure which welding standard you are using since you tossed SFA5.1 into the mix and then mention AWS D1.1, but here goes nothing.

Both E7016 and E7018 are included in both AWS A5.1 and ASME SFA5.1. AWS writes the filler metal specification, e.g., A5.1, and ASME adopts it for use for pressure vessels and pressure piping as SFA5.1.

Both E7016 and E7018 are limestone base electrodes capable of meeting the low hydrogen requirements for diffusible hydrogen, i.e., 16 ml/100 g of weld deposit.

E7016 differes from E7018 in the fact that E7018 contains a high percentage of iron powder, by weight, in the flux covering.

If you have to meet notch toughness, the classification of the SMAW electrode is an essential variable if you are welding in accordance with AWS D1.1 (refer to Table 4.6, item 3). If notch toughness is not a consideration for your application, the electrode classification is not an essential variable. The only thing you would have to do, if you are working to AWS D1.1, is revise the WPS to include both E7016 as well as E7018 provided notch toughness is not a consideration.

Best regards - Al