anyhow you will need to get the length of the mitten right.
If you change the decreases, get some orientation how many stitches you want to decrease and how long your mitten shall be. Then work in even rounds untill you want to round the top, do your decreases and then leave at least 1/3 of the stitches over on each needle in my eyes. Those (equal number on each side!) you can close by 3needle bind off or preferably by kitchener stitch. That would be one of the only ways I'd do a mitten: kitchener. (a video is up top!)
some mittens are very pointy on top, a lot of the scandinavian ones are! you can like or dislike the style. If you dislike: just adjust.
the type of needles is not responsible. The stitches end up being the same no matter which needles you use.