Use DFHack , and in the "choose your fortress location screen", on the DFHack console type
enable embark-tools
You will notice that it will add an option on the bottom of the screen :
s : enabled (none)
Press s and a menu will popup, press a to enable "Embark anywhere" and press enter.
You will now be able to embark on top of existing sites.
No idea for more dwarves, i remember in 0.28.x and lesser old versions when you reclaimed a fort you went in with a lot of armed dwarves (depending on the amount of enemies in your fallen forts), not just seven, this feature was lost starting with DF2010 unfortunately.
does this mean I can embark ontop of a dark pit/ fortress / town / necrotower and kill all inhabitants and take it for myself? how does this work with legends mode?
no idea about legend mode, but you can embark on those locations, just be warned that lag is horrible in most goblins sites in adventure mode and happens too with fortress mode :
by example :
Unfortunately, another thing to keep in mind is that df2014 (and still true in 40.24) npc/AI interactions are so messed up that most hostiles (and in dark fortress there are many) from gob sites (you can see them being hostile on the "U" list) will never try to kill your guys, and your guys will never attack them on their own.
By example i stationed my starting dwarves next to a goblin and there's absolutely no fight, despite i checked in the "U" list and that goblin is stated as "hostile" .
Now funny thing is that you will see gobs killing other gobs or trolls killing gobs or even the demon killing trolls and gobs that were spawned in his room for no obvious reason during your fortress attempt (and in the screenshot you can see broken arrows, it was a gob shooting at another gob around that location.
Only the demon lord on the top of the dark fortress will surely attack your dwarves if you get them into his room in the top of the dark fortress. But he will not bother to go out of his dark fortress and hunt them.
I really hope for his next version Toady will fix those AI/factions interactions from the complete mess they are in df2014, because the problem happens in caverns (and so no embark anywhere related) in which tribes of troglodytes and other kind of tribes living in caverns are all hostile but will never attack or be attacked when your dwarves walk right next to them.