The biggest technical hurdle with your proposed configuration is that the iMac only has the one Firewire bus... so, yes, you can get an adapter and run the 003 with no problems, you need a storage solution. In this case, a Firewire external hard disk daisy-chained to the 003 will probably be the best option - tracking to the internal drive is probably not the best idea. However, it's not quite as good a solution as a computer which can do a second Firewire bus via an expansion card (Macbook Pro 17" or previous gen 15" & 17" or Mac Pro). You may get some bottlenecking, basically, is the potential problem, but it will all still work fine - if your sessions get very dense and/or complex then you can say "I warned you," but for simple-to-medium-level complexity you'll be OK.
USB drives are not recommended for audio, although having said that I keep some sample libraries on USB disks on my Macbook Pro, but I don't record/track/mix from them. The data throughput is too stuttery for it to be reliable.
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