Fluidwill,
There is a free version of OA. Here is a comparison chart for the different versions, however you do not want OA++ which has an AV in it (conflicts with Avast): https://www.online-armor.com/products-online-armor-free.php. You can see my signature, and I have had no conflicts at all with my software.
Also remember that you can layer your security software, so that not every software does the same thing, but they work together for layers of defense.
I have tried other FW’s, and I can tell you to stay away from PC_Tools, which allowed a hacker to enter one of my machines even with other high security software on it. Other FW’s compatible with Avast are Comodo and Outpost.
Let me know if you have any questions. Thank you.