Fishing line is all I use to serve strings. I think this was 15 or 20 lb test. the spool wasn't marked any longer. I just stuck the spool on a bobbin winder on a sewing machine to wind the line around it.
Why not use Halo? Because mono is just as good in my opinion and that's what I have. Is mono better? probably not, but it's cheaper and, again, that's what I have.
I've used mono for years and have never had a serving break or come undone like some have expressed worry about. It's tough stuff and lasts forever. If you're in a situation where your serving gets nicked enough to cause it to break, you've probably cut your string..