Brown ghost knife stay a lot smaller (6-8") then black ghost knife (over a foot) and are commonly more docile. Your corys will grown around 2-3" and they could be able to co-exists with the brown ghost knife, but not in that sized tank. They really should be in a 55g, and I would say at 5" now is the time. If you wait much longer you might stunt its growth and shorten its lifespan dramatically. Also, I wouldn't recommend a redtail with cory cats. Redstails have a tendency to take over the bottom of a tank if there isn't anything larger/tougher then them. A 55g with a brown knife and redtail is a very good start for a semi aggressive tank. Most catfish spices would be ok with a redtail, but not cory cats.
As for the 29g, how long has it been setup with 5 gourami and 2 angels?