To make civilizations contact you sooner, you would need to add or alter the PROGRESS_TRIGGER tokens. You put in a number between zero and five. Zero will disable the specific type of trigger, while one-through-five are for different levels for the tag, with one being the lowest/least needed to trigger the civilization to visit, and five being the highest.
For example, [PROGRESS_TRIGGER_POP_SIEGE:3] would mean that a siege would not occur until you get eighty dwarves.
This will link you to the wiki pages for entities; specifically to the progress triggers, which can explain what level corresponds to what requirements.
There are a few ways to make creatures tougher. Such as making their tissues/materials stronger, making them larger, giving them natural skills, or higher than normal attributes. However, making a creature belong to a specific civilization is not something you can do through the raws alone. You can make it more likely for them to have the giant scorpions by allowing them to use any type of animal, however.
Iron access should be available to any civilization that has access to furnace operators as a profession, and steel should be available to any civilization that is permitted to use the steel smelting reaction.
Making undead similar to actual animated zombies may be a bit difficult, as zombies have their own behaviour that can't quite be put onto normal creatures, such as the ability to survive a bisection, but making an undead-like race that doesn't age or need to think, drink, or eat, while hating all life is possible.
There is a
wiki you can use.
In addition to the entity token I shared above, you may be interested in the
creature tokens.
And here is a reference
modding guide, which is also on the wiki.