"득실득실하다" means that a place swarms with people or animals. Not very positive image but doesn't escalate offensive things too much. As for the latter, I've never seen someone use "복실복실" that way in my life time though. Yes, you are correct for "furry", but people sometimes, especially young girls, would use some inadequate words unknowingly for their rhyme or something. Or she might have wanted to convey that the people's gathering seemed like a whole herd of sheep or gathering clouds. I guess that about does enough explanation. I'm sure you don't need to think too deeply about it.