The mylarai ability is "The Mylarai have the 'Enrich' Ability, which can be used on their planets. It increases the mining production of the planet by 1, and has a 40 hour cooldown and costs 200 energy to use" does that mean it can make a planet that has rich mining into very rich and keep progressing it every time the ability cools off or is there a limit of how many times u can use it on the same planet or total amount of planet?

Yes you can push the planet till it goes to 15x mega mining.

once you finish the mission that unlocks the race do you then get to change into that race for free or buy an artifact to change into the race?

If you have used up The Glaviz Device's you get from the zolazin mission chain and do not have any Sphere of the New Era from last new years. Then you will need to buy a race change.

i havent even got a galaviz device its from the zolalzin chain i just never did it yet

then you will have three free race changes when you do.