You can only have one food/drink buff active at one time. This puts a premium on the Blue/Rare foods (2 stat bonus) and Purple/Epic foods (3 stat bonus). I've seen the Rare foods recipes and have a few, but haven't seen Epic yet, so be on the lookout in trade guilds and snatch those up if you want to make bank come late game.
You can only drink one potion at a time yes, however you can use multiple buffs, such permanent mundus stone buff, food buffs, skill buffs, each type usable one at a time, apart from skills, which you can have as many buffs as you want from skills, unless of course they're the same, then they either overwrite or stack, depending on other variables.
But yes, after you drink one potion, it will have a global cooldown I believe, but to be sure I'll go check. [Edit] : definitely has a global cooldown on potions.