I am getting error messages when I build the DXT!

This is because you have not made the texture the same size as the one before it, it should be the same shape as the texture previously used on the object you cloned.

Does it matter if I use SimPE wizards instead?

SimPE wizards won't let you edit as much or in as much depth as SimPE itself will, but, if you are just starting out, then SimPE wizards is good.