If there is a tile 3/3/3 (with 3 essense) or 4/3/2 (with 2 essense) I build a city there and don't regret to lose a chance of building another city. If all tiles are with 0 or 1 essense I will try to build 2 cities. 6 grains it's too much. You don't really need so much. You can live normally with 3 or more grains. Especially if you have an orchard near.

Growth is not the most needed factor. Growth may be accelerated by many factors. But without Essense you will never enchant your troops produced in the city.

I would have chosen two 4/3/x over a single 6/3/x. Gives you better total growth, production and research - even considering the additional -3% unrest.

And I also (which may be bad strategy?) make three different city types early (Fortress, Conclave and Town). The tile with the highest essence gets to be a Fortress, so that troops can get more enchant-buffs (Aura of Grace etc). If I can make many cities I want one of each Fortress, Conclave and Town to grow quickly (after the initial Pioneer rush) - to get the benefits from higher lvl city. This means high grain tiles gets different city types on them IF possible. Why do you build 2 (or even 3, it's a bit unclear in your post) Towns first?

And as I play as a mage, I value Essence even higher than Production and Growth for early cities. Secondary I prioritize high production on the first 3 cities, to help build Pioneers and buildings quickly. 
 

I would consider a 6 grain tile great for 2nd or 3rd city, after that not so much. As mentioned by earlier posters, you can deal with lower amounts of grain as the game goes on. However for the early game,being able to get a quick Oracle, Infirmary or Scroll Scribe up early can be a godsend. (lvl 3 cities) They give 1 more Essence , more HP for troops, and Blizzard Scrolls (imba stuff when playing high level AI's) respectively. 

All except a town. My towns are high production ones since they have great food and growth buildings exclusively. Need Research? Conclave, Need good troops? Fortress. Towns are for junk sites and gildar mostly until later when you want +Faction Food for all your cities. Don't need high food for any of that.

Yes, 3 growth is the max you can get from surplus food alone.

I love to make fortresses on high food count tiles. You might pass up a good essence spot, but getting to prison and then onyx throne (both for global unrest bonuses) requires less food support from other cities, and 6 food helps the town grow at a healthy rate.

Please explain? Do you mean that higher city-levels gets higher research? Or do you mean that, given the same city-level and type, a Town level 3 with high population have better research than a 3-Town with lower pop?

Uhm, that's not my question.

I thought population (within cities of the same level) did not affect research in any way. But that city level affects research directly, = city-lvl x city-type's research bonus.

To explain/repeat better:  Can a level-3-Town with high population have better research than a level-3-Town with a lower population (given they have the same buildings and wonders also)? And if so, what calculation/bonus/building makes that happen?

Oops, sorry... you said it better.  City level (not population) x bonuses and unrest affect research.

Thanks Davronava and Trojasmic (I misinterpreted your text).