I've made me a little list of what effects bonuses have - the sins wiki won't help that much, because the names there are not the file-names of editor. I can place many bonuses on planets. Have set the artifacts and bonus probability to 0 and will place everything normally. I have planets with 9 bonus items - so it works.
For the metal/crystal asteroids:
If you create a metal/crystal asteroid, it will show in the gravity-well. But you have to edit its values. You find them, if you right-click on the green blob you created with a double click and then click 'edit'. Now, at the bottom of the list, you'll finde three lines to set values. They are set on 0, so you have to change that.
ExtractionRate: 0,35 [This is nearly the value like in-game] ( Imortant: There must be a , not a . )
RefineryRate: 0,25 [I've guessed this value - maybe wrong, maybe right, but it won't imbalance the game I think]