What you want to have is a separate drive just for audio. Your actual programs and plugins should still be installed on the main computer system drive, just the audio is recorded to the separate drive.
For sample libraries, you don't want them to be on the audio drive. If possible, you want them to be on another separate drive, separate from your system drive and separate from your audio drive.
Even in this case, the actual sample library program is still installed on the main system drive, only the samples are installed on the sample drive.
If you can't use a separate drive for your samples, then they can also be installed on the main system drive, but never on the audio drive.
Now that I've covered all that, you have a few options. For one thing, you can daisy chain firewire drives and an interface from one firewire port. I recommend these drives, which work great for this: OWC MEAQ7S1TB32M 1.0TB (1000GB) Mercury Elite Pro... in stock at OWC
You can get other sizes of those as well.
I have successfully run many tracks and plugins, plus streaming samples, off one firewire port on my 2007 Imac, with 3 drives and a 003 at the end. You should be able to do this unless you're doing gigantic composing sessions with orchestral samples, etc.
The other option would be to install a second drive in your Macbook Pro. This same company I linked to sells Do It Yourself kits with the drives included. They have videos on how to do this. Here's a page with links to different info: OWC 2.5" Drive and Enclosure Do It Yourself Bundles with insallation Toolkit
If you can do that, then you can just have one drive and your interface on the firewire port.