The tactic I use depends on how many planets I obtain at the start. If it's a small, low planet density galaxy specialisation is not the best way to go. If you grab a lot of planets you should definately have a few planets, preferably ones with special tiles, dedicated to factories so you can churn out huge ships quickly or a constructor every few turns.

When you do this the first time you'll notice your bank balance suffers badly. This means you need more cash and therefore some specialised economic planets. Higher PQ means higher population potential means more tax. The thing is, I like a high population on all my planets to help defeat any invaders that sneak through the fog of war. It makes no sense to waste all that potential tax so I normally build a few economic buildings to take advantage. This means I need 1 economic planet (with economic capital) and 2-3-4 industrially focused planets depending on map size. The same goes for research, 1 dedicated planet with research capital and disperse the rest evenly.

I agree mostly with C.N. Small planets research only. Use any bonus squares for their enhancements. Make sure you save room for tech/research/econ Capitals. Since people = tax revenue you have to build some farms. Don't be afraid to destroy some improvements to make way for others if your strategy changes.

Speaking of bonus tiles, that teardrop bonus for Approval...is that what you should be putting happiness things on?

if you are going to specialize a planet for, lets say, production and other planets for economy etc. if you want to have your production planets just pumping out ships, then this equals wasted production for all of the other planets. the ships would take so long to create, there is no point.

I've found that unless your population is getting really out of hand you only need 2 entertainment centers on planets that range from 12 to 16 in size and why 2 entertainment centers on small? (what do you consider small?)

Small lvl4~lvl8

And the second entertainment center is needed for highest xeno farm tech in combination with harsh taxing.

My smaller planets are usually either research or constructor builders. I use starbases pretty heavily so I'm always in need of constructors and they are relatively easy to build. Plus (I think still) military production doesn't carry over once a ship is finished, so using one of my heavy production planets for these can mean wasted production.

So one small planet in each "region" of my empire will be for these and the rest research.

Everything gets a minimum of 2 factories, although I will canibalize them later in the game when money is flowing easily and I can afford to buy more improvements.