Does a 6G SMAW welder legally covers all fillet weld under welding rule?
YES. Refer Section IX part QW 461.9

Is the welding electrode 7016 and 7018 under the same group?
YES. both belong to group F4. Refer Section IX QW-432 (Table)

Can it be a substitute for each other if the WPS requirement is 7018 only?
As per Section IX QW-253, it is acceptable. But ensure that your project specifications won’t put any restrictions to do so.