more Sens... not even kidding... since my 2 have gotten bigger my TT eel is out and about alot more because they are absolutely fearless buggers. I don't think w/ your stock list adding 1-2 more would be overload. if you had more aggro species in mind for the tank i'de hesitate... but if you just want someone thats not gonna be overly shy. my Bichirs are not shy...at all. And not posseing a threat to the cichlids will make them a good dither type fish to encourage them to be out 'n about.
Aussie rainbows aren't mutts, they're actual fish (well I suppose the Petsmart ones could be mutts). Not as pretty as Celebes or Bosemanis but they're solid schooling fish.