If i go on a minecraft pc server with mods on it do i have to have the mod pack?

While game servers may have rules regarding what CLIENT mods (mods you install on YOUR copy of Minecraft), most of them will usually "play well" with SERVER mods. Content mods, like Pixelmon or Technic stuff, will likely cause more issues than more simple mods (like OptiFine & MiniMap mods) since they limit content conflicts.

HOWEVER, you will ALWAYS want to check with the server admin / staff PRIOR to utilizing mods to stay in compliance with their rules.

With the current Minecraft launcher permits multiple versions, you can keep a vanilla copy on hand to start with prior to switching to your client-modded copy.

No, you don't. Not sure with this other guy is talk about.