You might want to check out the following website on Germans and slavery in Texas:
http://www.wtblock.com/wtblockjr/texas.htm
According to this history of German immigrants to Texas, few of them owned slaves. In my own research in Louisiana, I have also come across very few slave owners who had German surnames. I transcribed the entire 1860 Rapides parish (Louisiana) slave schedule, and of the over 500 slave owners, only 5 were German immigrants. The largest slave owning immigrant group was Irish.