Mayweather wouldn't walk through Morales or Barrera, but it wouldn't be a highly competitive fight, and Mayweather was faster than Manny at 130, his hands were faster, and he got off way quicker. Against Manny you can see everything coming a mile away, or you could it was just to fast for you to react anyways. Mayweather has that type of speed but he has always had the ability to go from 0 to 60 quicker than Pacquiao, his training and style allow for that. Also Mayweather was a natural 130 pounder which means he wasn't afraid to mix it up and throw combinations unlike nowadays against way bigger guys, and once again styles make fights. Floyd would start peppering Morales with jabs and bring the warrior out in Morales at which point FLoyd would be able to completely control the fight with his vastly superior speed and better accuracy and precision.

If Barrera and MOrales both possessed really good defensive skills or were great counter punchers I might be inclined to agree that they would give Mayweather problems, but they weren't quick enough off the gun to catch Floyd coming in and out.