Goblins are bugged, can't start with pick
Ah, the embark profile itself is bugged. It should give them "tunneler" instead of pick. I bet there are other civs with this issue; i'll review.
Every civilization has a pick, but some of them have it listed as either "mattock" or "tunneler" instead. I may remove this, the mod has outgrown it; but anyway everyone can start with a pick. Let me know if tunneler isn't showing up for goblins, that would be a problem.
Where can I find chaos witches / pyromancy slab?
That's actually two different questions. Chaos witches are in the Magma Sea; they can swim in lava so they may come up to your fort from magma smelters or magma pools. The config file c_variation_highfantasy also give you the option to let them spawn on layer 3, which may or may not be the only way to see them in adventure mode (I'm not sure, can you visit the magma sea in adventure mode?)
The pyromancy slab would be carried by pyromancers, who are secret-keepers that don't build towers (same with miracles secret, silencers, and druid/nature secret. Not all of these have slabs; some of the are just master and apprentice). Pyromancers are never towers, but whether they are slab or just master/apprentice is something I need to refresh myself on. I suspect just master/apprentice, so it may be quite hard to find one.
One thing that IS bugged is chaos witches and other creatures dropping foci as a kill reward. It doesn't currently work right, but I think I've got it fixed for v1.1.1 eventually. (it was Extra Butcher Object, but you can't butcher sapients...now it's an "unusual corpse" in 1.1.1 instead).
Merfolk civs are too big
Interesting, I've never had that problem. For example Save10 has 9000 mer vs 40000 dwarves, elves, goblins. And as Plump suggested, the sahuagin and naga are supposed to help contain this. I'll make sure that both sahuagin and naga have "can expand over oceans" which should help more.
If you want to disable Mer, they are the first entity in the file entity_highfantasy_modded, which also contains sahuagin, halflings, and hobgoblins.