If it was me I would keep the 20g because you can never have enough tanks! Using the water from the 20 won't really help the 55 anyway, the BB isn't in the water. You could, like me set it up without a filter. Have a read up on the Berlin Method of filtration. Of course like many things there are variations to the rules
I think it's important to use all live rock and lots of it to begin and although strictly speaking the true method is a bare bottom I did end up putting a very shallow sand bed in.
I run a powerful skimmer and have 2 x powerheads. I do 20% PWC
weekly. My parameters are stable and always have been, nitrates never more than .5
It was also a quick cycle, about 2 weeks.
Hope this helps!