Choosing a license doesn't limit you (the author). It's setting a default for what others may do with your work. Anyone is always able to contact you for different licenses. For example you can distribute the code under the GPL license and also sell proprietary licenses. You can change the license later if you choose.

The biggest difference is that the BSD license does not ensure that users of third party libraries are able to improve the code. The purpose of the GPL is to guarantee that.

There are no issues in meshing GPL and modified BSD code unless someone is trying to prevent users from using the source code.