There's absolutely no need to do a Full Scan every night, not even every week, avast or any other active AV scanner looks at every read and write, the only reason to do full system scans is either when you install the AV or for the eventuality that you got an infected file of a new virus (not present in the definitions at the time) and the later definitions might caught that, for this eventuality, and if you are really paranoid, once a week should be enough*.
As for the warnings, these are just related to the inability to decompress and scan inside those compressed file, these can be safely ignored.
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* - Honestly, if you are this paranoid then you should know that no AV catches 100% of the viruses even older viruses, there's no 100% guarantee of safeness and full scans every week do very very little to increase your safeness. What increases it substantially is being careful with what you download from the internet.
