I considered running hydra briefly, but shyed away from the extra complexity and operational overhead. Most things are already available in the community hydra, and building the unique things is hardly an inconvenience since it happens once per machine. Instead of using channels, we maintain a fork of nixpkgs and occasionally squash it into a git subtree of our main repo. Our git history reflects everything you could want to know to replicate a working build of our software.

I've never done a gc on my /nix/store, and have wrecklessly added things to it while experimenting. It's 25GB and has 44k (!) derivations/packages in it.