I'm with Knot, do what you want on your bike. And keep those reflectors (unless they look like they'll fall out).

However I'm not sure "last rider" meant you could join. I would recommend against joining a paceline without being explicitly invited. Riding a paceline can be very dangerous, and requires a lot of trust in one's skills, and in those of the others. Outsiders are normally not welcome, in my neck of the woods at least.

If you join a club ride, disclosing from the start that you're a beginner with group riding will help, they're going to teach you the basics.

I don't know how your speakers are holding to your bike, but you might not want to keep them on during a group ride if there's even the most remote risk that they would fall (or that anything else would fall off). It would also probably not be considered polite to have music on during a group ride.

Ride on!