(1) Clothing is not the thing you need.
There're CLOTHING and SUBTERRANEAN_CLOTHING and they accordingly spawn creatures in cloth and silk items. You may place one of them or two so you'll have variety of things or just made from usual cloth or just made from silk.
I don't know what will be if you remove it completely...
But the easiest way is to remove clothing items directly from the goblin entity.
Open the file "entity_default.txt". You'll see a lot of [WEAPON:***], [ARMOR:***], [PANTS:***] etc. Remove the things you don't want (i.e [PANTS:ITEM_PANTS_PANTS:COMMON] and [SHOES:ITEM_SHOES_SOCKS:COMMON] etc. Leave them some armor (platemail, greaves, helm) for some challenge and free iron after ambushes for you if you want.
(1b) FAT just determines how much fat can be extracted from creature. There's no settable parameter as "toughness" but their size affects their defence/damage a lot. Also there's a DAMBLOCK token (look at dwarves, they have it = 1, I think because of body hair) and it resembles natural armor (like hard skin/scales).
Plus you may give them [STOUT] token thus they'll be much less likely to be knocked down.
To reduce insta-deaths you may want to tweak body to be better (i.e. I made 3-parts upperbody: chest, right torso side and left torso side, so there's no chance to hit 2lungs+heart with 1 little dart).
(2) Entity placement is totally random.
(3) You may adjust the total goblin civ. size though. Look at the same entity_default.txt file. You'll see at the end of each entity tokena like MAX_STARTING_CIV_NUMBER - it shows how many different civs of this race are spawned in the 0 year, also there's MAX_POP_NUMBER and MAX_SITE_POP_NUMBER.
MAX_SITE_POP_NUMBER - allows you to set how many creatures can live in single town (max limit).
MAX_POP_NUMBER - sets maximum population for the whole world.
You may adjust these values to be relatively bigger to make sure there'll be more goblins generated.
Also note that the creatures sometimes may settle in 1 town so if you have MAX_POP_NUMBER 100000 and MAX_SITE_POP_NUMBER 100000 there's a chance that they'll live all in single town so make the 2nd parameter lower.
It's easy to make wizard race, I think Sean did it in MA mod and I have White Wizards and Dark Sorcerers in my middle earth mod.
What's with Zillion elves? I didn't try it.