Have some fun with it. I would go with:

2 - black Banded Leporinus 

10 - tiger barbs

2 - Pictus spotted catfish

1 - Albino Bristlenose Pleco 

one option - 

15 tiger barbs, you could mix albino, green,regular and even glo - tiger barbs for variety. 

And a bristlenose or other dwarf pleco. and or a rainbow shark

another option-

12 odessa barbs

and 12 giant danio

dwarf pleco or rainbow shark

or - becuase you are looking for semi aggressive non cichlids

8 celebese halfbeeks, 2-3 males, the rest females

they are top level fish so you can add a middle dwelling school of your choice.  maybey 6-8 turquise rainbow fish and a rainbow shark.

or - 

8-12 American flag killi fish, 2-3 females per male

12 cherry barbs

6 yoyo loaches or zebra loaches

or 

1 male and 3 female mollies which will multiply

10 lamp eye tetra

6 pictus catfish

Tin Foil barbs will get quite large in a 55. And they are semi aggressive. 

Marble cat fish will get huge and just by their size they can be considered aggressive-they are mean looking too. I had 2 of them in a 75. I had to get rid of them due to their size-they got to be about 12 inches long and 4-5 inches around. Just their movement through the tank would harm the other fish. 

I have had jelly bean parrots in my tanks for years. And although they are considered 'cichlids', they are man made fish, and rather calm-but they also get quite large-and they change colors are they grow.