Permissions are a pest.
Basically, you need to make sure that all of those developers can write to everything in the git repo.
Skip down to The New-Wave Solution for the superior method of granting a group of developers write capability.
The Standard Solution
If you put all the developers in a specially-created group, you can, in principle, just do:
chgrp -R <whatever group> gitrepo
chmod -R g+swX gitrepo
Then change the umask for the users to 002, so that new files get created with group-writable permissions.
The problems with this are legion; if you’re on a distro that assumes a umask of 022 (such as having a common users group that includes everyone by default), this can open up security problems elsewhere. And sooner or later, something is going to screw up your carefully crafted permissions scheme, putting the repo out of action until you get root access and fix it up (i.e., re-running the above commands).
The New-Wave Solution
A superior solution—though less well understood, and which requires a bit more OS/tool support—is to use POSIX extended attributes. I’ve only come to this area fairly recently, so my knowledge here isn’t as hot as it could be. But basically, an extended ACL is the ability to set permissions on more than just the 3 default slots (user/group/other).
So once again, create your group, then run:
setfacl -R -m g:<whatever group>:rwX gitrepo
find gitrepo -type d | xargs setfacl -R -m d:g:<whatever group>:rwX
This sets up the extended ACL for the group so that the group members can read/write/access whatever files are already there (the first line); then, also tell all existing directories that new files should have this same ACL applied (the second line).
Hope that gets you on your way.
The documentation states that the directories under SVNParentPath have to be repositories, so no looking through all subdirectories.
You might be able to make script though, that would output several Location-blocks matching the parent folders of SVN repositories.
Best Answer
I only use Git, so I wil try at least to help you with that:
If you do not see any trouble administrating your repos via the command line, gitosis should do the trick pretty nicely.
If you do really need a web interface, you could take a look at repo.or.cz (http://repo.or.cz/w/girocco.git) or gitorious (http://gitorious.org/gitorious). Repo.or.cz is uglier, but it is way easier to install (gitorious is open-source, but it is also the software powering gitorious.org - they do not have much incentive to write nice instructions)
Here is a more comprehensive list of options: https://git.wiki.kernel.org/index.php/GitHosting
Anyone of those options will give you easy creation of new repositories.
Now, one word of caution: you should never, EVER, use the unix server user list for repository permissions. It is easy enough to mess with, and results are easily catastrophic (gitosis uses a simple file configuration, and ssh keys. Should do the trick for you).
Another thing, I don't see why you need to have subversion, HG and Git repositories. Most projects use only one of those options. Care to elaborate why?