Short foot print like that you don't want too active of swimmers.
I have would fill it 3/4 of the way full and do 6-8 Archer fish. Make it a biotope with driftwood and live plants in and out of the water.
Short tall and wide. Hmm.. I'd do a large hard scape resembling something from the Amazon and stock with 6 angels, 2 GBRs, 12 or so otos, and an L series pleco.
