These snails are a useful part of a tank's ecosystem. They are only a problem if the population gets out of control and the main reason for that happening is over feeding the fish. Left over fish food = lots of food for snails. I have these and those tiny ramshorns in both my tanks, and I hardly see them as I am careful not to over feed the fish and shrimps.
For my first 10 gallon tank I made it a male guppy/endler tank! guppies can be any color so they’re super fun to search for while exploring new fish stores.
Since you had koi originally, you might like the color variety of guppies ~
I would recommend getting a group of at least 5 males and avoid females just because guppies breed wayyyyyy to easily