Each user can choose her homepage by clicking on the house-shaped icon at the top-right of the page. This icon doesn’t appear on all pages in the UI, but it is available on Projects and IIRC on Portfolios in 6.7.
And it is incumbent upon me to say that 6.7.* is past EOL. The current LTS is 7.9.2 and the current version is 8.2. Normally I would end this by saying you should upgrade at your earliest convenience. However, I’ve just realized that in 7.9 we accidentally dropped the ability to set a portfolio as Home, so you might actually want to wait just a little longer.
As for modifying the interface, no that’s not possible.
Update: You actually can set a Portfolio as Home in 7.9.2. So, yeah. Upgrade at your earliest convenience.