Oxydoras niger will grow to 3 feet / 90 cm no matter your tank size and with many more years will reach 4 feet / 120 cm in a large tank. The 650 gal will be too small for it sooner than you think.
The g-cat may stay at 2', so might actually be adequate in the 650 gal. I wouldn't say splendid. Just barely adequate.
thebiggerthebetter, I don't want to imagine the day I will have to say goodbye to my Oxydora niger! Specially here in Portugal, where it will by very difficult or impossible to find a bigger tank to house my loved fish...
I started to do a daily water change of abaut 25%. Will always try to keep this fellows in the best conditions I can.