Good questions David,
First the answer is no. Getting your ATP certificate has become quite a procedure In the past it was just a written and a checkride. Now there are simulator requirements, a plethora of flight time requirement and weeks of additional training all of which costs thousands. The Regionals don’t expect you to have your ATP because they know full well how difficult it is to get so no it wouldn’t be a disadvantage. They do require you to have the required 1500hrs so that you can get your ATP certificate. Once you’re hired the Regional will do the training and your type ride will be your ATP checkride.
Now is it advantageous to train with ATP? Absolutely. Why? Because the airlines know that ATP students have demonstrated their ability to be successful in an accelerated training program similar what they have. Want proof? If you’re an instructor with ATP you can interview with a Regional with only 500hrs and receive a conditional offer a full year before the guy on the street. More proof? More proof? A number of Regionals contract at with ATP to do THEIR ATP certificate training for their newhires. If they trust ATP to train their newhires they clearly have faith in the organization and their training.
Not trying to sound like a salesman but those are the facts.