As Frank said they are fine to be left on your system, they only use resources when running.
I would suggest a periodic running of these additional tools as a back up to avast. Start them, go on-line and get the latest signatures, disconnect from the internet, pause the Standard Shield provider and then run the other on-demand scans. Once complete don't forget to enable the standard shield again.
Doing this ensures that only one scan is in progress (with no interaction with avast), this not only speeds up the scan duration but avoids any possible conflict if both avast and the other scanner can detect the same malware.