[OS X TeX] Installing unstable packages

[Theo Moore]

Hi,

I'd like to install the pgfplots unstable package
(http://pgfplots.sourceforge.net/). I already have the latest stable
version of pgfplots installed, but I need a feature from the unstable
version. I have a couple of questions:

[1] Is there a (painless) way to install both and use one or the other
when the situation demands? I have lots of documents that use
pgfplots, so it wouldn't be nice if installing the newer, unstable
version somehow begins producing errors in the older documents.

[2] Where should the package be installed? There were a couple paths
suggested in (http://www.mail-archive.com/lyx-users@lists.lyx.org/msg67001.html),
and it seems most of them will work equally. However, I know I have
the older pgfplots package: what should I be doing to avoid conflicts?

Thanks!