I'm planning on tracking source with bzr but will be needing to include some clones and checkouts of other projects mine depends on.
Is there any advice beyond simply adding their directories to an ignore file and moving on?
You can check out the git and svn branches with Bazaar into the appropriate directories (using the bzr-svn and bzr-git plugins). Then you can update or push everything in one go using the bzr-externals plugin.
But if you just wanted to manually use git and svn then yeah check it out with those tools and ignore the directories.