Almost all other antivirus cannot be installed with another antivirus, it's because the antivirus designed for main protection in your PC. It's not the case for Smadav, Smadav is an antivirus that is designed as additional (second layer) protection, so it's mostly compatible and could be installed and run together with another antivirus in your PC. Smadav using their own technique (behavior, heuristic, and whitelisting) to detect and clean virus that improves security in your PC.
Smadav has advantage with its very small installer size (under 2 MB) and low usage of internet while active in your PC. And also smadav only using small fraction of your PC resources. Smadav most of the time only use small memory and small CPU usage. With this small resource usage, Smadav will not heavily slow your computer. And you can still install another antivirus that will together work with Smadav to protect your PC.