Signal Messenger currently IS working on my LOS and has been successfully registered for GCM/FCM, but had issues before. You need to make sure microG is correctly configured before installing any apps depending on it. For Signal there are 2 options: Using GCM and therefor enable it in microG before your install the app or using the websocket connection, which may have an impact on your battery life.
Stadtrad and K9 are working flawlessly on my device currently. I think K9 doesn’t require any of the microG stuff. Stadtrad needs Google Maps, but that’s working fine. Didn’t install DB-App yet, because I’ve been using it only infrequently before. But there may be issues with push notifications if you rely on them.