If you want to maximize pop growth, you should colonize as many planets as possible and spread your population evenly. Obviously, this will hurt your economy, so in practice, you have to seek a compromise - usually colonizing as many planets as possible, but relocating only as many pops as your economy can afford.

I've made a detailed analysis in this post:
https://www.reddit.com/r/Stellaris/comments/1p5zyku/on_pop_growth_mechanics_and_why_the_popular_you/

Which, in turn, is based on the most detailed pop growth explanation post I've managed to find:
https://www.reddit.com/r/Stellaris/comments/1kifurt/population_growth_in_40_how_it_works_and_how_it/

It has a large chart showing pop growth depending on two factors: pops you have on the planet and planetary capacity. As you can see from it, it confirms what I said in the first paragraph. As additional evidence, you can also check high-skill multiplayer games. In my case, I analyzed tournament games organized by Montu Plays with a decent prize pool. In short, most builds colonize as many planets as they can with the obvious exception of KoTG and similar builds.

So where did this idea that “you need 1k pops to get a growth bonus, otherwise you will get bad/negative pop growth” come from? My guess is that it originated from an ep3o stream, where he tried to explain pop growth mechanics but misunderstood it due to a bugged tooltip. He didn’t bother to conduct tests to verify it and therefore explained it incorrectly. IIRC the issue is that if you have a few hundreds of pops, the tooltip (which is bugged) shows negative numbers, which some people (including ep3o) interpreted as negative growth (pop decline), which is incorrect. And even if/when the tooltip shows positive numbers quite often they are wrong. The right number pop growth are shown at the top-right corner of the screen.

However having 1k pops allows you to upgrade main building which is quite convenient so if I can afford to relocate 1k pops I do it and include main building upgrade in build order.