yes, you can use the mid-pipe with any other aftermarket muffler for the most part. I recommend you don't use the stock muffler if that's what you had in mind. Also, if you get yourself a straight thru muffler, I suggest you get a stromung resonator b/c no aftermarket muffler is as good sounding as Stromung's.
for the cat here's the deal...I think I had an issue with it but I'm not 100% on whether the tank I removed was the cat or not (I'm almost sure it was the cat). Anyway, I haven't had the opportunity to look under the front end of my car to re-check. Nevertheless, I believe whatever issue I faced only applies to my particular model. So therefore I believe for the most part that the following cars are safe from finding any issue with the cat:
B13s, b15s, 200sx's, any SR20 powered vehicle, and probably the 98s and 99s.
Selrider99, I'm sure you wont have any problem and kristinspapi, you probably won't have to worry either.