Is there any story completion percentages that you gain for doing any of those side quests like Publizing a company or finding a bride? Or do those lead to situations later on that will gain you persentage?

Find a Bride - Nothing. No story completion.

PR - It's good if you start it now, but just make sure to finish it in Chapter 5 for Episode Complete / story completion. It gets harder to finish it the longer you wait, but you can always finish it.

How would it get harder? You can't get to some of the people you can publize to or there's a time limit or it just takes longer if you don't spread the publicitly through all the chapters?

If you don't get enough PR points by talking to people across Spira, you must lose credits (to the appropriate company) to gain the points you need.

It's much easier to just win people over throughout the chapters.

What BetaWave said and kinda what you said. People disappear and appear throughout the chapters and you can promote each person once per chapter.

If you really want to be hardcore about it, though, you should try to get over 400 points for both companies in Chapter 1, or at least by Chapter 3 if you can. That maxes both companies at Level 5 in Chapter 5, unlocked all special games and items and gaining Episode Complete.

Yeah, you can pitch to everyone person once per chapter. So that makes you able to pitch to every person 5 times, but actually it's 4 times ('cept for the people at the Moonflow; you'll find out why). You should start ASAP cause you need 400 pts. for one company and each chapter would give about 300, so you need to promote in two chapters (not including Ch.4 cause that gets you like 35 pts.)

Each time you promote to a person and say the correct response, you get 5 pts. Saying the wrong response will get you 0-3 pts., depending on the person.

Hope I helped a bit.