Believe it or not, Deb, size is not always the determining factor in herd structure and hierarchy. If your horses have done well with other horses in the past, they will likely do well with this new pinto.
The procedure you are using is what is often called a “howdy.” The animals have the opportunity to see, hear and even interact a bit while still in between a fence. In most cases, when this is done with horses over a couple of weeks, the herd accepts the newcomer easily when in the same pen. Watch how the three behave with each other around the shared water tank. Do they pin their ears? Spin and threaten to kick? Does the pinto avoid interaction, or does he initiate interaction? You may want to set up some feed buckets close to the tank to observe their behavior when food is present.
The only other thing I would suggest is enlisting some friends to come take all three out on a few trail rides together. Allow them to stand next to each other, bump into one another and so forth. Good luck!