Potty training is one my boys' greatest strengths/achievements, lol, so I won't tell you.
Rudy's size and the issue of relations with Myles may be playing a little into your challenges with potty training him. Neutering may help, but don't be discouraged if there is a temporary setback. There is a small chance that things could worse before they get better.
Keep up with frequent trips outside, focusing on going out immediately after meals, immediately when waking up for the day or after a nap, immediately after play session or any other excitement, and right before bedtime.
Keep up with the praise.
Maybe Rudy also needs a different system to alert you that he needs to go. Maybe bells on the doorknob for him to paw and ring? There are many choices.
As for relations between the two boys, that can take time. I brought my second Yorkie home at 8 months old, but he was still an annoying puppy to my nearly 2 year old. There was also jealousy from the older one, Max. Some things we worked out fairly quickly, but overall, it took a good 8 months before things were perfect.
You can encourage your boys' bonding by doing activities together that both like, maybe walks or having a special treat together.
Sometimes, unfortunately, a much older dog never accepts the puppy.