Two pointe to make here.
1) Not all regaments have Ratlings or Ogryns. Your right, thay aren't trusted or even tolerated by many in the Imperium of Man. And thats a part of the 40k world. Same with Psykers (Sanctioned or not), thay have no place in some regaments. BUT ,There are millions of cases of severel regaments joining forces to combat a comman foe. And thay may bring there Ratlings, Ogryns and Psykers to the fight for a tactical advantige over there foe. Just remember 1 thing ,YOUR PLAYING PROFESSIONAL SOLDERS. If ordered to work with abumans THAY WILL OR BE SHOT BY THE COMMISSARS FOR INSUBORDINATION. And, If thay had no place in a guard army, then thay would have bin killed to the last by all humans.
2) Ratlings and Ogryns don't have to be played by the hobit and simpleton. Im in a campan that the ratling is a conniving little SOB with a sniper rifle. And the Ogryn is trying to be a big game hunter by learning from the ratling. So theres 2 that are braking there molds. It is 1 of the fun things you can do buy making a great backstory. So don't think that theres only 1 way to play a race or class.