I am guessing you are talking about a 4 cell flooded lead acid golf cart battery? Like the trojan T-875, maybe? I know flooded and AGM batteries have slightly different charging requirements, and I do not think it would be advisable to mix different battery types. I have not researched it, though, so that is only like, my opinion, man. That being said, the voltage mismatch will rule out a 12 volt system anyway. If it was me, I would consider setting one of the 8v batteries aside and make a 24 volt system with the other 3 that was completely separate from your current setup. You would need to get more stuff, but you would get redundancy - which I have found is nice to have when you are your own power company. I have only had the “grid” go down once in the last 5 years (and only 2 customers were affected by the outage).
As for the fridge, I would highly recommend a chest freezer conversion. You can buy an external thermostat that people use to make beer brewing fridges when they want a temperature range above that of a normal fridge. You can get them for 50$, and then plug in a small 5 cu ft chest freezer with the thermostat set to 38 or whatever. That is what I have been using, and it works great. In the heat of summer when its 90 degrees out it will still only pull about 200 Wh per day.