I've seen videos where people have some how made every sim clickable or watchable(not sure which), wherever they are in town. It has something to do with the Nraas mods, and the only video I found was one that showed it being done, under guise of showing you HOW it was done. This is what I wish to know, I think it would be useful especially to see who was coming into your town and stuff like that.

I've not seen the video, but it sounds like NRaas Tagger to me. You can find it here http://nraas.wikispaces.com/Tagger

ETA: Once you fall in love with it, it's impossible to play without it.

To suggest an alternative solution, you can also try an older version of Story Progression.
However, keep in mind that your patch level is crucial; make sure to get the version that's appropriate for yours or you will have issues.

IIRC sim tags were separated from SP to Tagger in the 1.67 version. I'm at 1.63 and have no intention to jump to 1.67. Using the SP meant for 1.63, I can still see sim tags and can select which sims I want to display tags, an option that Tagger doesn't have at the moment. I'm referring to the Blood (i.e. family), Romantic, Spouse etc. options, not sexual orientation, age and all the other options Tagger offers.

In SP: General Options -> Options: Sims -> Display Map Tags (or something like that)

Is there a way to set tags for all sims, or do I have to do it manually for each I see?

All sims will have tags by default; you can filter out those you don't want to see.

Just FYI, filtered tags will be available again in the next version of Tagger currently in beta testing.
http://nraas.wikispaces.com/Tagger+Testing

One of the reasons map and lot tags were separated from SP was to improve stability and control SP-induced lag. SP has more than enough to do already and those of us with very busy, over-crowded worlds find that we don't need to turn SP's overall speed down nearly as often, or ever in many cases, with the tag function being controlled by Tagger instead.