that's great. There are members on here that can accurately interpret all of those figures, unfortunately I'm not one of them.
But let's go with the 'soft water'. That is actually really good as there are way more soft water fish than there are hard. Hard water fish are mostly all of the live bearers, so that's - Guppies, Platies, Molly's and swordtails. There's also African cichlids on the hard water fish list too.
So, soft water fish - all the tetra's, harlequin rasboras, central and South American cichlids, corydora's, catfish, betta's and many more.
For this 54 litre I would look at harlequin rasboras (including the dwarfs), the smaller tetra's such as neons and cardinals the smaller cories such as panda's and pygmy's
All of those fish need to be in a group of their own kind so if you choose neons for instance I'd go with a group of 10 neons. If you decide on harlequins I'd go with a group of 10 of those. If you decide you would like something for the bottom to go with the fish in the upper levels that's fine as they inhabit different area's of the tank. You could add a large group of pygmy's or go with 6 of the smaller cories such as the panda's I mentioned. There are plenty of small sized cories other than panda's but panda's tend to be readily available. Some of the other smaller types tend to more specialist.
This tank isn't big enough for a plec but once it's been running more than 6 months you could consider adding a small group of oto's to clean the algae off the glass/plants/decor
Hope that helps