A niger cat is a 4' catfish, so not a niger, imo. Plus they grow super fast as well. When I think nigers, I think 1,000 gallons.
An irwini for life? I'd probably not do that (but that's me), however I'm sure people have done it with good housekeeping. The 30" width might work if it's not blocked off too much with decor. I love 'em both, but in that tank, the irwini is a much better choice.