It's Nightwish..
There will not BE a 'mosh pit' if by 'mosh pit' you mean 'crowd of twats trying to kill each other'.
There might be a group of slightly less creaky people right at the front and they might bounce around a bit, it'll be fine.
The average age of a Nightwish fan is going to be 40+ so I wouldn't worry about getting standing tickets, most folk will want to sit down!
Do stand, you don't have to stand at the front, the back and sides will be safe as houses, but seated it just isn't the same experience at all.
On the whole I've found the goth and metal scene and in fact any of the supposedly 'scary' music genres to be the kindest and friendliest going, particularly where the average age of fans is into middle age.
I have had some unpleasant pit experiences (you don't want to fall down in one, with several hundred coked up Audioweb fans whose sole mission is to actually kill someone) but you don't have to be there and if you know, front, centre, is where it's going to happen, you can avoid that easily enough.
Whilst I am here, parents can you educate your younglings that the mosh pit is where you can leap around and dance and no one minds if they get the accidental elbow to the face, if someone crowd surfs over them or stage dives and its where a circle might happen...
A mosh pit is NOT a free for all opportunity to inflict as much physical damage on a stranger as you possibly can.
If someone goes down, you pick them up rather than stamp on their face, if someone loses their specs, ya find them and hand them back rather than smash them to bits on purpose.