1. What are spheres?
http://dwarffortresswiki.org/index.php/DF2014:Sphere2. How do I make a certain creature take over goblin civs, like HFS?
[POWER] token will do this, in theory. However, in order to take over a goblin civ, they (or someone else) must get rid of the demon first.
3. How do I specify how many of a creature is created during worldgen?
For regular animals, [POPULATION_NUMBER] will affect the average number per map tile.
For civilized creatures, it's tied to their entities, reproduction rate, and tendency to die.
A set number of megabeasts and semimegabeasts are created, based on worldgen parameters. Semimegabeasts are more common.
Night creatures are, as far as I can tell, always unique unless they manage to kidnap someone to breed with.
4. How do I make a creature attack forts that have a certain amount of wealth, leading an army of lesser creatures, also like HFS?
[ATTACK_TRIGGER] affects when they will show up. Only civilized creatures and necromancers can lead an army. You can create a necromancer secret that transforms corpses into a "minion" creature when it animates them, allowing you to make custom wizards with specific creatures as minions. However, their behavior can be a bit buggy.
5. How do I make a creature gen semi-randomly, like FBs and titans?
Can't.
6. How do I make a creature reproduce on its own?
If you mean asexual reproduction, you can't. There is a trick to making creatures spawn other creatures with an interaction, involving transformations and necromancy, but this is rather advanced stuff and full of bugs in any event, so I wouldn't advise it if you can help it.
7. How do I make a creature be made out of a certain material?
Look up the Bronze Colossus for a good example of this.
8. How do I determine the speed of a creature?
GAIT tags.
9. How would I make a creature able to reanimate corpses like a necromancer without a slab?
Create a custom interaction that is basically a copy of the EXAMPLE RAISE you'll find in raw/interaction_examples/interaction_secret.txt. Then make the creature able to use that interaction.
Read about interactions here.
10. How would I change the butchering yields of a creature?
Make the creature bigger?
11. When making a sentient race capable of sieging, how would I change what mounts and war animals they use, the numbers they attack in, and their general strategy, if possible?
[SIEGER] and [AMBUSHER] will affect strategy.
[USE_GOOD_ANIMALS] will let them use animals with the GOOD tag.
[USE_EVIL_ANIMALS] will let them use animals with the EVIL tag.
[USE_CAVE_ANIMALS] will let them use animals found underground.
[USE_ANY_PET_RACE] will let them use PET_EXOTIC and MOUNT_EXOTIC creatures by default.
The numbers can't be affected directly, but if their population is higher their sieges will tend to be bigger. This is tied to reproduction rate and how often they tend to die.
12. Can a creature survive if it's made only out of one type of body part (for instance, a creature made out of 50 eyeballs)?
Yes, provided you give it the necessary tags [NOBREATHE], [NOTHOUGHT], etc.
13. Is it possible to make a creature cause tantrums and insanity?
Interactions can apply syndromes, and syndromes can add thoughts and alter moods. You can also use a syndrome to add the CRAZED tag, which will cause them to attack everything in sight; basically like going berserk.
14. Is it possible to make a creature immune to dragonfire, regular fire, and even magma?
[FIREIMMUNE] will make prevent them from catching on fire instantly when hit by fireballs, firejets, and magma, and [FIREIMMUNE_SUPER] will work with dragonfire as well. However, remaining in a place with high temperatures will still cause their body materials to catch fire or melt, even with these tags. For this, you will need to change the heatdamage, ignite, and melting points of their body materials. Look at the bottom of the dragon raws for an example of how to do this.
15. Is it possible to make a creature to give birth to another creature of a different species?
No. Well, yes, sort of, but you would need to use the above-mentioned spawning trick.
16. How do I make two creatures of a different species not hostile to each other, if possible? (like HFS)
BENIGN creatures will generally not attack unless provoked.
AT_PEACE_WITH_WILDLIFE creatures will not be hostile to any creature with the NATURAL tag.
Also, any tips you expert modders may have about making a creature very powerful and likely to survive long would be appreciated. I'm going for something around the lines of "eldritch horror".
Plenty of ways. But the cheapest, simplest method (and the one most likely to be respected by worldgen) is just making them huge. Size DOES matter.