Hello im new here joined coz i read your plans for an african tank and just thought id throw my experience out there!
unfortuantly i feel congo tetras may get a litle big and way too active for a 180, need to be in large groups and i wouldnt put em in anything smaller than a 300 because of this. The very minimum u need a mixed sex group of 6 and you might just get away with it but good congo tetras are not cheap and may not be worth the risk, maybe african red eyes may be more suitable?
Also the spotted bush fish grows to 6inches and gets territorial so the suggestion of 3 or 4 may be abit much. maybe just one.
Other fish that do well are senagal bichirs grow to nearly a foot but one may be okay in your tank as theyre not too active. a large (at least 6)school of true upsidedown catfish would also be awesome wouldnt get both tho each needs its space. African vampire shrimp grow to 6 inches and would be a suitable tank mate. also maybe a couple more butterfly fish as they enjoy the company of others and as a result are alot more active.
One thing i would mention are kribs, when breeding they are highly territorial and will take over ur tank. they wont bother the butterfly but will batter most other fish, theyd take on my 8inch syno eupterus's to the point that they would just hide constantly. Also bear in mind that this is in my 500ltr with only a single breeding pair of kribs, in a 180 theyd take over. I have however been tempted by the blockhead cichlid (steatocranus) as they are very unusual and knowhere near as fast and agile as kribs but being cichlids expect some aggro when breeding (must be kept as a male and female pair or they will get depressed and waste away)!
I know you dont want to hear this but i feel the usual african oddballs need a bigger tank. knife fish, elephantnose etc, Ive got em all and personally a 300ltr is absolute minimum.
My suggestion would be, 6-10 african tetra (if u can find anything abit smaller than the congo tetra, i know of african yellow fins and red eyes but dont know bout size or activeness. If u do get congos i would get 6 no more no less due to its social needs and ur limited tank space), another butterfly or 2, 1 spotted bush fish and 6 or more upsidedown catfish!I tend to think of upsidedown catfish as the african corys, always wizzin about together, dont get big either! The bush fish then becoming the centerpiece, they have a great personality comparable to cichlids and are very friendly with their owner.
Alternative to the upsidowners, a senagal bichir or a pair of blockhead cichlids maybe? (never kept blockheads but their behaviour has always facinated me) any of these alternatives will require a soft sand substrate and caves, upsidedown cats just need plants and wood to hang around but need to be kept in groups to be happy and active.
Also the vast majority of african fish require quite subdued lighting or you may never see them from hiding, floating plants do the trick there!
African fish are amazing and unique, im planning a monster sized 850gallon 8ft beast of a tank, hoping to capture a slice of the congo in my livingroom.
Some things to read for some african ideas....
http

/www.practicalfishkeeping.co.uk/content.php?sid=4153
http

/www.practicalfishkeeping.co.uk/content.php?sid=3616
A vid of my current african community tank, got most things in there! as said its a 500ltr 110-130gal depending on what part the world ur from, but got a 850gal planned as my puffer will grow to 2.5ft!
http

/www.youtube.com/watch?v=COkOoOSaog0&list=HL1339332659&feature=mh_lolz
Its a lively biostrous tank but no outright aggro anymore since i split up my kribs! Just ignore the java fern, couldnt find african fern anywhere! May not work on ipads n things due to content restrictions.
Hope you have as much fun as i have had with african fish! expensive but soo worth it!
Any questions please ask.
Ed
P.s the above post is talking complete sense any african fish = much larger than ur average bioload for some reason, maybe its just they love their food. and also water quality is of great importance the majority of the african fish are very sensitive and will not tolerate anything other than the most pristine of water conditions (elephantnose for example). However on the flipside those bichirs in my experience are tough as old boots and live forever!
I'd just disagree with the kribs, individuals are fine but mine were some of the most brutal bullys of fish ive ever seen, they wanted that tank to themselves and nigh on got it by battering everything else into hiding no matter how big the fish was. No damage was done tho, all my fish can handle their own if needs be!