Who Gives A Fish Forums

I think you'll be alright with the kribs, they're not as bad as some people make out, and your tank's a good size so they should have plenty of space to themselves.

20 ppm nitrates isn't too bad, but the Frogbit should help. Just keep an eye on your water params and adjust your light if needed. Too much light plus nutrients is basically an algae party invite.

Floating plants can definitely help by shading the water and soaking up excess nutrients. I've had good luck with Duckweed in my 10 gallon, but it can get out of hand if you don't thin it regularly. What's your nitrate level like?