Well I have been using Outpost for many years starting way back on the outpost free firewall 1.0. But soon purchased the Outpost Firewall Pro on version 3.0 on a lifetime license deal (couldn't resist). But they also do a free suite
http://free.agnitum.com/, the old stand alone free firewall isn't offered.
However, when installing the free suite it detects avast and asks confirmation that you have avast installed, answer yes. It won't install the AV, web control or anti-spyware modules to retain compatibility with avast.
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- Whilst Outpost would have you exclude its folder from avast scans, I don't feel that is required, if outpost is running in its compatibility mode with no AV, anti-spyware and no web control modules running.
Personally other than a single file exclusion I don't believe you need exclude the Outpost folder, I haven't in all of the time that I have been using avast and outpost together and that is many years.
This is the file and path that I have in the File System Shield, Expert Settings, Exclusions section, C:\Program Files\Agnitum\Outpost Firewall Pro\wl_hook.dll (easiest it to copy and paste this full path). This file is as its name implies a hooking tool that outpost uses to monitor activity also and is the only file where I ever had any issue with avast.