The issue is that the element content of planets is highly affected by collisions. And the collisions have a huge amount of happenstance involved. In particular, the type of collision is important. A glancing collision tends to blast away all the lighter stuff and leave most of the heavier elements. A direct collsion can produce a number of different effects from the two joining up, to both getting pulverized, to them exhanging material and going on, depending on the relative speeds and the exact angle.
So Mercury seems to have been the result of a glancing collision. The debris wound up getting absorbed into the sun. The Earth-Moon system seems to have been more direct, but with the two objects exchanging material and the smaller becoming our Moon.
So you can pretty much dial-up how much metal and heavier elements you want in a planet. Within the range of what Mercurt has to what Mars has. You just choose which collisions happened. You can also dial up whether they are larger or smaller within the range, by having the extra mass blalsted off into space or absorbed by the two colliding objects.
There will be limits, of course. But those limits will be exceedingly difficult to pull out for any specific stellar system candidate. It would require a huge amount of computer calculations to determine if a particular set of planet sizes and metal contents could have resulted from a plausible series of collisions. As long as things don't look too drastically different from our own solar system, your audience will have a difficult time knowing if it is impossible. Probably you don't want a Jupiter-sized planet in the place of Mercury, for example.
Otherwise, you can just dial up what your story needs.
