Disable Touchwiz launcher on a rooted Samsung Device
There's no need to uninstall the Touchwiz's launcher or to install a new ROM. The solution to this particular scenario is to disable the package asociated to the Touchwiz's launcher APK (Thanks to @AaronGillion for pointing out its name and location).

To do this just execute the following command as root in the ADB shell:

pm disable com.sec.android.app.launcher


In both, Samsung Galaxy Tab 3 7" wifi (Android 4.1.2) and Samsug Galaxy tab 4 7" Wifi (Android 4.4.2), the previous command worked correctly.

Having my custom launcher installed but having selected deliberately the Touchwiz's launcher as the default launcher, after disabeling its package my custom launcher was set automatically as the new default launcher. 

Just as a side note, notice that I had the same results in both tablets even though on the Tab 3 I installed the custom launcher as a regular app and on the Tab 4 as a system app (/system/priv-app/).