In AIX 5, /opt is the "official" preference and should be a separate filesystem (though it doesn't do any harm if it isn't). However, there isn't really a right and wrong, so there's nothing wrong with a /usr/opt; but if you want to tidy up then go for /opt.
There are freeware packages supplied by IBM, and others, pre-compiled. www.rootvg.net
has links to some websites; or the IBM Linux applications CD (which ships with AIX 5.1 and 5.2 CDs) has lots on too, from bash and KDE to python. It all works fine, though may be somewhat down level. Also, it seems to me that the rpm that ships with AIX 5.1/5.2 has a bug where it doesn't pick up on dependencies automatically so you need a bit of messing around to install some things in rpm format.
If you want the latest versions, you'll want to build from sources; but it isn't necessary.