Stocking - 64 Litre 60cm Tank

Rams might be OK if you kept the water at 26/27 deg C. That is right at the top end for the barbs but just about right for rams.
With soft acid water you'd be fine with apistogrammas, if you like the looks of those. They like the same temps as cherry barbs. The most common apistos in the shops are cacatuoides (cockatoo apistos) and agassizi.

If you had sand, I would suggest a shoal of one of the dwarf cory (pygmaeus, habrosus or hastatus).

I would normally recommend bolivian rams, but as they are slightly bigger than 'ordinary' rams, a 64 litre tank might not have a big enough footprint for them. My 60 litre was 60 x 30cm and seriouslyfish recommends a minimum length of 80cm for bolivian rams.

Kirstos, can you tell us the measurements of the tank, particularly the width and depth (that is, front to back) please. For bottom dwelling fish, the size of the tank floor is as important as volume. For example, my 50 litre tank has the same footprint as the 60 litre it replaced so can have the same amount of small bottom dwellers as long as I don't have as many fish in total.