Besides the regular neighbors on the embark map i got a unfamiliar one "Mountain halls". What is this exactly? Are they friendly or should i fear them even more then the necro-towers? The DF wiki gives is mysteriously little information about them other then being other dwarf fortresses.
Mountain halls are simply dwarven-built settlements. Having one close to your fortress will not impact you in any meaningful way, perhaps with the exception of larger migrant waves due to close proximity if both settlements belong to the same civilisation. I don't believe it is possible to start out at war with another dwarven civilisation due to wars being sparked by conflicting ethics, which are defined per race, not per civilisation, so you should be quite safe from the short menace.
Occasionally a mountain hall may be conquered by another civilisation, and if they are hostile to you you may suffer (or enjoy, depending on your ability or level of masochism) increased attacks, but it is relatively uncommon.