Forget what you know about indiviuals, Mycelius don't populate a planet, there is only one Mycelius on a planet, a single fungus which grows to cover the hole planet.

They or to begin it doesn't use ships to colonize other starsystems, millions of spores so small they can attach to a photon are ejected out of the atmosphere and ride with the light to the planet of choice.

As fungus they don't know war, they just want to grow as is in their nature, the fungus are able to communicate with everyone who had contact with the spores.

Features:

-Colonizing neighbouring Systems without a Colonizationship (perhaps unable to build them)

-Grows without the need of agriculture (grows regularly, I think in tendency slower than the others - growth period depending on the planet, fast on terran, jungle, ocean, slowly on tundra and desert, even slower on lava (and the other one) and nearly no growth on gasplanets)

Feature idea:

-Starting with a fully populized planet, but you can only eighter build or research (for each planet)

-Symbiose, the fungus isn't overgrowing the planet, so every population can be filled twice, (ofc you get only % of the ressouces the other species produce and he also gets % - those % change more in favor of the owner of the system/planet) (faction population is not influenced by you choice between research and industry)

-"Telling your friends" you get a diplomacy bonus with a faction for every population on your planets

-You can only build ships of other races (expect the smallest) on planets they live (you can only build what the others have as design, but can be upgraded as they upgrade) - you have to do this and this to let you build their bigger ships.

- Diplomatic option for letting them "colonize" your planets and the otherway around

Story (narrated by the Mycelius on the homeplanet)

I found my self, perhaps in a spiritual way, but mostly in a physical way, I found my other end. The planet grows on me you could say, I am the planet. I've become able to read the fauna, which walkes on me and they look up to the stars. I could go there, spread my spores through the universe and end my lonelyness on this rock. Find more fauna to read the mind and dream further. 

I am Mycelius the sole of my art but this won't stay like this for ever and maybe only maybe I'll finde something the fauna on my planet calls a friend.

Gameplay:

My basic idea for the early gameplay is that you have to find a balance between build an research, it's easy to populate other star systems but you eigher build stuff on it or do research, so you need to have an eye on your researche speed, while trying not to have underdeveloped colonies. Being only able too build the ships of your allies means, that you aren't a militairy superpower just by yourself. Symbiose - having other factions on your planet gives you a small amount of fids they produce and let them have the rest or parts of it (balance question not sure). On the other hand when you "land" on a planet of other factions they get a part of your "fids" and you have the rest. 

I know the balancing is a real problem when you can have double the population, but I think there is always a way to balance it out.

I hope there aren't to many faults, you are invited to politely correct them.

Also I am open for ideas of others for further developement of my concept.

*Edits:

1- feature idea: bio-ship

2- growth depending on planets

3- not all planets together build or research, each one of the can one or the other

4- explaination ships in the gameplay.

5- delet of ship growth

6-feature idea: Symbiose, "Telling your friends", the warhips of others, "diplomatic colonization" + explaination in Gameplay: