you could add more rbps to the 55g as long as you are able to upgrade in couple years to at least a 75g. the additional 6" width will make the p's a lot more comfortable.
if you are able to got with a 125g I would suggest more than just 6, 20g per fish is just a general rule that some follows but some don't, especially if you have tank bigger than 100g (depends on footprint of the tank)
assuming the filter in your 22g is capable of handling the bio load with the fish you can, you actually transfer all the fish to the new tank as long as you also add the old filter from the 22g to the new tank. the bio load won't increase and you have more than double of the water volume, even adding one or two more rbps it shouldn't impact it by much. you could always add some stress zyme and/or stability (from seachem) to boost the bacterias. I've never had problem with those products..
if you are able to got with a 125g I would suggest more than just 6, 20g per fish is just a general rule that some follows but some don't, especially if you have tank bigger than 100g (depends on footprint of the tank)
assuming the filter in your 22g is capable of handling the bio load with the fish you can, you actually transfer all the fish to the new tank as long as you also add the old filter from the 22g to the new tank. the bio load won't increase and you have more than double of the water volume, even adding one or two more rbps it shouldn't impact it by much. you could always add some stress zyme and/or stability (from seachem) to boost the bacterias. I've never had problem with those products..