I currently have a decent set of speakers, the Yamaha NS-SP1800s, but I'm not completely satisfied with the center. Would I hear a noticeable difference with the POLK RM8 center? The CS series speakers are way too big for me so would the RM8C be good enough? Or if you can suggest another similar(small) one I would greatly appreciate it. Thank you.
You would hear a difference because the Polk won't match the Yamahas.
Not a good idea to mix different brands of front speakers. If you like Polks, get 3 RM8 Polks up front. If you prefer your Yamahas, keep the matching center for your SP1800 and perhaps adjust the volume level of the center channel.
Both these sets of speakers have small 2.5" drivers. You might just need bigger speakers to get better sound.
But it is your system so do whatever sounds best to YOU. I'll never hear it.