custom piping for exhaust?

A full cat-back is easier to install and usually gives better power and sound. Some people save money by getting a universal muffler and having a shop do the piping.

but it will still sound good and loud with the custom piping or the stock piping?

If you plan on just welding a universal muffler onto your stock piping, you won't see much in power gains. I can't say on sound because most universal mufflers sound like crap, but not all. Getting the larger custom piping will give you more performance and deeper sound. If you're really worried about it, spend the extra money on a Greddy or Tanabe cat-back. You'll be much happier.

i already ordered the apexi n1 universal muffler and im deciding whether to have it welded on stock pipe or get the custom 2.25" pipe

Get the custom pipe.