Not chemically; 6mg is 6mg either way. It feels stronger in the throat because unprotonated nicotine is harsher on the way down, while salts smuggle the same dose past with less sensation. That is why salt bottles go up to 20mg and freebase rarely sells above 18mg.
Anyone on a sub-ohm setup, where low-strength freebase is the standard; vapers who miss the throat hit that salts smooth away; and anyone stepping down strength, since the freebase ladder has more low rungs. Fresh switchers from cigarettes usually do better starting on salts and revisiting freebase later.
Yes, they blend fine, and it is a handy trick: a splash of 18mg freebase lifts a 0mg shortfill, or half-and-half of each softens a harsh mix. Keep track of the resulting strength and shake properly; beyond that there are no rules against it.
