Re: one breast per feed or two?
It is very normal for weight gain to slow somewhere in the 4-6 month range. As the baby becomes more mobile, he starts putting his calories into reaching/rolling/scooting/crawling/etc. instead of packing them on as fat. So I would not be concerned about the slow weight gain, particularly if your baby seems satisfied at the breast and is producing sufficient wet and poopy diapers.
That being said, it's very normal for a mom who needs to block feed in the early months to eventually transition to doing 2 sided feedings. It doesn't sound like you need to, but at this point it's probably worth a try.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"