I hate to say it but ur asking for trouble. Congo's are not suitable for your tank mates, think bigger fish that congos wont pick to shreds. They get very large for a tetra 4in+ (ive seen a freak one close to 6inches) they have a decent set of teeth on them perfect for picking and shredding up fins. They need to be kept in larger numbers so they keep their boisterous semi-agrro behaviour to themselves and not pick at other fish. 2 fish will not work and i'd worry for your other fish. Trust me, ive been keeping congo's for years, they are like mini piranhas, if i put a guppy in there it wouldnt last 5 minutes! (we'll it wouldnt have any fins left thats for sure!)
They are beautiful fish but unless kept in large enough numbers (most people seem to think 6 as a minimum) they can have quite a mean streak.
I have around 30 of these beauties some ive had nearly 10 years now, awesome fish, voracious feeders!
dont do it. Get yourself some more ottos and cories if you must add more fish, they could do with more company!