there are times where im not sure if you have to alternate pick or down pick a certain melody

for instance take the famous guitar riff of the beatles day tripper, its played during the intro and verses. anways are you supposed to do all downpicks for this melody or are you supposed to alternate pick depending on whether the note is on an on or off beat. (go down on the 1,2,3,4 and go up on the and's) , thanks.

also another question for people who read sheet music with tab, lets say you are reading a rhythm chord chart for a song, if there are eigth notes and sixteenth notes in the same measure (and the follwing measures) then the eigth notes will get down strumes and the sixteenth notes will be alternate , right?
ya but even if you alternate pick your still following the rule of down picks on the on beat and up picks on the off beat, right