Sorry for that, i meant planet development research as you corrected (top left, not at home both for OP and this).
Thanks for your info, will use it to GREAT SUCCESS!
I've always been one to max that upon getting the planet, will fix that issue up quick smart now.
So i'm going to assume that i shouldn't bother with refineries unless they're on a planet with many connections or no viable trade routes?
If you know you can hold the planet and will be to busy to go back and check the population cap, then getting all the development upgrades at once isn't a bad thing. It is just getting rid of the underdevelopment penalty (that causes the negative credit value of planets early on) that you at least have to do. Increasing it beyond that point (after the second upgrade) just increases the max population but does not increase the population growth rate. So the later upgrades don't help until after the current population cap is reached.
You've got the refineries right, but any planet with logistical slots can have a "viable trade route". Even if it doesn't increase the trade chain it will still generate the same amount of credits as every other trade port in your empire without planetary bonuses (unless your trade ships try to go through an enemy planet and get destroyed quickly). The bonus from a long trade chain is applied globally to every trade port in your empire (which is why extending it can bring generous payoffs), so even if that planet is in a completely different solar system, it will generate the same amount of credits as a trade port on your homeworld that is in the middle of the trade chain, because both gain the credit bonus from that trade chain.
Also, I should mention that early game the best method expanding your economy is to simply get as many planets as possible. Plenty of people have run the numbers and acquiring planets, building extractors and getting the early population upgrades are by far the most cost effective way of increasing your income. The fact that your research base and ship production are also somewhat based on the number of planets you control make this even more vital. Do not start investing in trade ports or anything else until you can no longer expand to neutral planets (or enemy ones if they are lightly defended).