Hello & welcome back - Sorry to hear you had tank-headaches there for a while
Based of the current stock am I assuming correctly that you have soft acidic water there?
I'd honestly not recommend mixing barb's & Tetra's, Barb's are much more active and/ or agressive and that with senstive Tetra's is not a good mix....So I'd pers go one way OR the other and I'd more then likely make this decision based off you water.
Here's what I'd do where this my tank
Add 4 more Serpae and add 3 more black skirts
Not sure exactly what "congo" you have...maybe a Phenacogrammus interruptus
aka Congo Tetra, could you googel these pictures and see if that's what you got? Don't wanna wind up making suggestions on the wrong fish here
Do your homework on the shark matter first, many of them you'd commonly find at the stores will outgrow your 55g quick and are also aggressive enough to snack down your Tetra and I'm not sure that's what you'd wanna see there.