What are you trying to achieve?
Do you want to find more metal types in more types of stone layers? Such as magnetite in obsidian layers?
Do you just want to find more useful ores versus the less useful ores, while keeping all ores restrained to their vanilla stone layers?
Do you want to add MORE minerals and things to the existing game?
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NOTE: :SMALL_CLUSTER: might not be the true tag name, check your raws.
Now, some general things you might want to consider: First, change all of the :CLUSTER: (maybe leave it on bauxite and magnetite if you want) tags to :SMALL_CLUSTER: or :VEIN:. This will remove those huge patches of randomly colored rock (alunite, gypsum, olivine and microcline to name a few) and replace them with smaller patches/veins. In my experience (not much) I _seemed_ to get more veins of ore/useful stuff when I converted all the :CLUSTER: tags to :SMALL_CLUSTER: (this may or may not have an effect on gem populations, so I've taken to just removing the annoying stones you find all over from the game entirely).
Another thing you might consider (depending on how world gen works) is duplicating the important metal ores (hematite, b coal, etc) in the raws, so you have [MATGLOSS_STONE:HEMATITE] [the other tags] followed by [MATGLOSS_STONE:HEMATITE2] [the other tags], etc. From what I gather, the various ores and things compete for places on your map at gen(less likely)/embark(likely) time.
So if you have a gabbro layer, and obsidian layer and a dolomite layer (ha!) on the same map, the various matgloss_minerals will get selected at embark. All of the possible minerals (and gems, mind) that can appear in gabbro will be sorted through, and the game randomly(?) selects them to occupy spaces on your map. lots of map squares get the big huge CLUSTER minerals, and then the game throws in some veins, and then scatters gems all over the place.
So by increasing the number of [MATGLOSS_STONE:HEMATITE2] entries in your matgloss_mineral file, you should up the odds of the game placing that instead of some other vein. (This is all hearsay/best guess. I'm still playing around with gens and adding/removing matgloss entries, and don't really have anything concrete)