Arches is a paper commonly used, but Rives BFK is a very easy and almost fool proof paper to use since it is a silkscreen paper that takes well to water-based mediums.
fabriano is a great option too. guoache looks beautiful on rice paper. guoache looks beautiful on everything. gouache looks like 70's black light posters on black paper
if you really desire a smooth, opaque, bright surface, which is the beauty of this paint, pre mix your colors to the consistancy of melted ice cream, you want the paint to flow but not to be so thin that you can see through it, add a touch of white to any color you use out of the tube, just enough to give it some body but not so much that you aren't getting the color you desire. rinse you brush frequently, use sable, don't be a schmuck, the brush quality really does matter. after rinsing dry your brush on a paper towel, so you don't keep ruining the consistancy of your paint. you have to find your way with it, its really a personal matter how you get the stuff to go on smooth. its really fussy, but very very beautiful...
Dried gouache can be activated again by putting water on it. This can be helpful if you like to mix colors on the paper, but it's a pain when simply trying to "lay down" new colors on top of each other or fix mistakes. Mix up a thick amount of paint, load your brush up, then try to lay down the new layer in as few strokes as possible ( Acrylic/gouache hybrids are available now and make this problem history if it's really a bother).
bleed proof white, available by the inks and dyes in most art stores, is the traditional solution you can simply apply it on top of the dried paint let it dry then paint over it.
there are a few colors that will stain paper regardless of how hard you try to get them up, any of the red violet colors: magenta, allizarin crimson, rose tyrien etc all stain paper and with time may actually bleed through any color you apply on top of them. bleed proof white on top will solve the problem.
if you do make a mistake you need to fix you can carefully swab the dried paint with water (don't get it on the paper too much), then take a roll of toilet paper and gently with pressure roll over it, picking u p a layer of paint. remove used paper from the roll and continue. this can really take a while but if its something special you have made one mistake on, you probably don't want to ruin the surface of the piece by globbing over it with paint.