Yes, you can run two or more libraries on Itunes from your Macbook. Where you keep the seperate libraries is inconsequential.
As far as changing the file size on the fly, I don't think that will work. If I understand, you'd like to store music in an uncompressed format such as .AIF and have it change to .AAC or something similar on the fly? Why would you want to do this? Itunes will simply play the uncompressed file from the external HD without taxing your Macbook.
As far as changing the file size on the fly, I don't think that will work. If I understand, you'd like to store music in an uncompressed format such as .AIF and have it change to .AAC or something similar on the fly? Why would you want to do this? Itunes will simply play the uncompressed file from the external HD without taxing your Macbook.