As said before, you can use any combination of drives you like, as they are independent devices.
I was going to say that in response to Kar98, replacing the MBP's internal hard drive with a 7200RPM drive will cause you to have a shorter battery life (worth it for the performance increase if you ask me 😀), but it looks like you already have a 7200RPM drive in it. If you do opt to replace the internal, Seagate is a good choice. I have four Seagates in my desktop of varying capacities and they work great. They also have a long 5-year warranty. A few months ago I bought a 200GB SATA drive from a friend for $20 only to discover it did not work. I told him, and he gave me the money back but said I could keep the drive and try to fix it or whatever. Curious, I entered the serial number on Seagate's site, and sure enough, it had about year's warranty left. I sent it in for an RMA and 1 week later received a 300GB absolutely free.
If you get an external, I'm a fan of LaCie drives. They look great and perform well.
In regards to your last question, any mainstream (read: recognized brand) USB or Firewire drive will work but I would research compatibility anyway.