Lifeforms are another cost sink for Empires that are really meant to be done after you have gotten your initial infrastructure in place. Namely once you've gotten Mines, Solar Plant + Fusion Plant(if desired), Robotics, Shipyard, RLab, Nanites and other infrastructure in place along with requisite Research(ex: Plasma for Mine output and Plasma Turrets). Nothing says you can't get your presence up to Tier 1 levels or 22 Farm/21 Residence to unlock the Lifeform Research Center and do Tier 1 Technologies since the costs for those aren't too bad but the impact from Lifeforms is not noticeable until you have 9-10 Planets and have managed to get Lifeform Technology with similar effects(ex: Volcanic Batteries + Geothermal Power Plants) amongst several planets to level 5 or so to amplify their impact. The further on down the Tier you go, the more costly it is to upgrade. Namely Volcanic Batteries is way cheaper than Geothermal Power Plants despite both impacting the same thing at the same rate per level. However, while they may appear to be a waste now, if you plan on KEEPING said Lifeform Technologies, they'll have an impact, however small, the moment you research them. You just won't really feel them until your Empire grows much larger and/or you upgrade the Lifeform Technology to the point where cumulatively it adds up to like 10%. Also remember, Lifeform Technology effects stack so when one Planet boosts Energy output by 8% and another 2%, you effectively get 10% globally so doing Lifeform Research isn't a waste. Any Lifeform Buildings you build are exclusive to the planet they are on so keep that in mind.
Yet to answer your question, I'd focus on traditional stuff first. Lifeforms honestly can wait unless you got nothing better to spend your resources on.
Lifeforms, AFAIK, all have the same population caps for their respective T1 Residential building. How much Food their respective Farm produces and hence can support should also be similar but could vary depending on faction. What does vary by faction is food consumption/growth rate. Some factions grow faster than others and Rock'Tal is notorious for being slow for growth. That may also apply to starting population but really does it matter if you have 100 or 250 to start? You'll end up with millions on the planet eventually.
Sorry - I mean the 'start spaces'/Planet size of the planet where you can build, not population...
Ahh that's completely different and depends on what slot within the system you colonize. Being a Discoverer and/or in an Alliance with Research specialization can affect the outcome to generate bigger planets. What "roll" you get for a given spot is completely random and sometimes players have to abandon and try again. Slot 8 will yield you the largest planets and a whopping 35% boost to Metal production. This guide will help out: Colonizing Guide
Also note that Slots 6-10 also yield a bonus to Metal Production(albeit at lower rate) and Slots 1-3 yield a bonus to Crystal Production(Slot 1 being the largest). Deuterium is based on Planet Temperature(Colder it is, more you get). Conversely, the hotter a planet is, more energy from Solar Sats.
Thank you.
I know about the different slot(e.g. 1-15)
I would be happy about a size of 220 and above. I got that in first or second try when I had Humans as lifeform.
Just very strange since I moved to Rock'tal, I am down under 190 - 5 slots in a row - all on slot 8.
Then you got below average rolls and Lifeforms have nothing to do with Planet Size outcome. I have 4 planets, two on Slot 8. One is 302, other 320. Granted I'm a Discoverer with Alliance Research for a 15% boost but you need to set your sights on bigger planets if you can afford to do it. It'll pay off later down the line.
Great - I will try the discover for a time being, do some explorations as well 
