Make sure it doesn’t fall into any storm drains. I saw a kid get dragged into one by a clown before.
Is it talking to its rain coat, or someone off screen?
Love fluffies that enjoy water. 
I’ll go splash in puddles with that fluffy. 
I still jump in puddles in the rain during the summer if I get the chance. Did so on my way home from work having borrowed a raincoat 3 sizes too big for me. There were a lot of cars which mysteriosusly swerved after driving past me. It might’ve been due to what they thought was a short kid in dad’s raincoat was in fact a fullgrown man with a massive beard! 
Whoever put that coat on him better also give him some water wings. Just to be safe.
I honestly have no idea how Pennywise would react to fluffies. On the one hand, they do scare easily. On the other hand, they are woefully stupid and have very short attention spans, so if they see a ‘munstah’ and survive, they will quickly forget about it.
It didn’t normally target cats and dogs, I’m not sure it would target fluffies either. Except perhaps out of sheer irritation.
And now I’m imagining a particularly clueless fluffy following It through the sewers talking nonstop.
“Nu fwend have pretty bawoons. Ew, nu fwend home nu smew pretty, wots o bad poopies. Fwuffy see wittwe boatsie, whewe date come from? Wat nu fwend do to wittwe humie? Why booboo juice?”
Glamours such as Pennywise feed upon fear.
And it’s ridiculously easy to horrify fluffies
but unfortunately their little minds break far easier than those of human children, ending the fear too soon.
So Pennywise would have very little use for fluffies, unless it simply couldn’t find children to feed upon.
Such was my thought, yes. In the book they speculate that It must have fed on animals before humans arrived, but afterward It fed exclusively on humans (and mostly children, with the occasional adult thrown in.)
It also seemed to cause anger and violence to skyrocket, and it may be that it fed on that as well, or it might just have been a side effect.
It is a fascinating villain, especially the few times King shows us what It is thinking, but sadly oversimplified in the recent movie.
Careful playing splash splash now