There's no way to toggle anything in macros. I really don't think you need to tell party members when songs are on or off in most cases though since the ranges on them are pretty wide and typically since you are a BRD and can stand wherever you want, you should be the one moving if needed.

Paeon is mostly useful in sections of fights where you have to AOE and have several physical DPS, like the first and 4th waves in Turn 4 or if a physical DPS has been raised as that sets their TP at 0. You typically don't have to leave it on long at all, a few tics is usually enough to get peoples' TP back to manageable levels. As Viridiana said, this is mostly done by request or after practicing something with a group you can figure out when you will need to use it to recover TP.

For Requiem, I just keep it up full time if it's a short fight or it isn't very MP intensive. If Ballad will need to be used at any point during a fight or you are running with randoms, it's probably safer just to keep Requiem off until it's needed.