Im no expert, but I just added an apple snail (skipper)
He's so cute and fun to watch. When I first put him in, he "ran" all over my tank eating brown algae, poor guy, he must have been starving. He has slowed down on the algae consumption though, I think he likes progold and bloodworms better
My goldies don't bother him at all, Just make sure you get one bigger than your goldies mouths
I've been reading about siamese algae eaters. They are supposed to be way more peaceful than most of the other algae eaters, and actually eat algae

.
Just make sure it is the real thing. there are lots of imposters ie siamese flying fox. They are also supposed to be hard to find, but luckily my lfs has them. (a family run shop)
but I did op for the snail though, I may have to rethink this if the algae gets out of control.