Noir_CZ: What I aim to create is to create a systém with other players (AI) that I won't have to bother with them as I would need just single fleet to blockade them all and if lucky most of the planest would be toxic or tiny :-)
One of the problems you will run in to is that when an AI race perceives itself as too weak, it surrenders to another AI race. SO especially if you are blockading their last planet and that planet is quite weak, a surrender is quite likely.
Noir_CZ: is it possible like to have a system where already another player is, get one planet there and colonize it ...
Yes, but you can only colonize a planet in an enemy system when there are no enemy ships present, or if you have a NAP with them. Your colonies start with a single pop count of your own race, so you will have to send assimilated races to the planet manually if you want the population there to include alien races.
Noir_CZ: ...and then give it via diplomacy to another player?
Yes, but but you need to have contact with the race you wish to give it to, and friendly enough relations that they speak to you. Afaik the AI doesn't seem to move around assimilated population. So whatever the racial mix it is likely to remain the same once you give the planet away.