i used a piece of cloth around my intake to slow down my filter. on another tank I put a clear piece of plastic under the outflow across to the other edge of the tank so the "waterfall" was spread horizontally over the remaining surface and didn't create a downward "draft"
my betta was always stressed with filters in tanks 10 gallons and under, i did half day shifts so he had sleeping time with no current. With a 5.5 you may not have that long and you are doing a fish-in cycle, which I did not have to contend with.
The good news is plakats have less finnage which if I ever get more bettas I will prefer. Many long finned bettas bite their tails in high current, like mine!@
Good luck with the baby! so little! dont over feed her, what are you using, pellets?
my suggestion: soak all the foods in her water and smush up big pieces
do you have a heater? keep it warm-- my man is sensitive to cold also, he was a sickly cup-bound fish and I barely kept him through many trials the first 3 months. Now he is super fat and happy in a big tank with neons and red cherry shrimp and ramshorn snails, and I spoil him