I find that the "on-off" status of the hiss is more noticable than a constant level, and I don't get drop offs in the harmonies that way. The purpose of a gate is to limit the signal strength that enters the mic. This (in theory) keeps out unwanted noises that are below the designated threshold and leaves room only for the intended vocal signal.
The problem is: sometimes the gate will close on a soft vocal passage and screw up your phrasing and dynamics.

Compressors are equally tricky to set correctly, and will definatly cause troubles with dynamics if not set properly.
I still advise to leave them off for the most expressive response from the mic.
Read up and practice setting levels before you decide if they will help you or hinder you. It's an easy choice for me ..... I get a much better sound using the mic and vocal techniques that I've mastered over time. A machine can't think creativly ... it only works with mathmatical settings... sometimes good....most times inappropriate.

Try turning both off and see what you get.