F8 Puffer and Fancy Guppies in a planted 21 gallon long UNS rimless tank?

[Pooka]

Hi, I've kept planted freshwater tanks for 7 years and I'm planning to start my first F8 tank. I will greatly appreciate any advice regarding the tank or tank-mates. The LFS currently keeps their F8s in freshwater with a pinch of salt so I will measure the salinity of the store's tank when I buy the F8 and then gradually raise the salinity in my 21 gallon tank until it's around 1.005. I originally planned to keep two F8 puffers in the 21 gallon tank with egg crate cover and a 40 gallon HOB filter. If the F8s start to show aggression towards each other, I would place a divider in the middle of the tank. Now, I am considering getting just one F8 along with some sweet looking fancy guppies being sold at the same LFS. Can fancy guppies handle a 1.005 salinity? The F8 puffers at the LFS are already pushing two inches so they may refuse to coexist with the guppies... but c'est la vie. I will try to keep them satiated with live brine shrimp, snails, and gammarus from my live-food-farm. Follow up question: Can lipstick gobies coexist with F8 puffers in a brackish tank?

[Pufferpunk]

You cannot keep 2 F8s in a 20g tank. A singleton is recommended. Puffers love to bite long fins like guppy's. Also, can you keep a top on a rimless tank? Your puffer WILL jump!
library/puffers-in-focus/fig8/

[Pooka]

Thanks for the advice! I have an egg crate light diffuser cover that is too narrow for the puffers to pass through. It will allow for easy feeding and water top-offs. Do you think evaporation and salt creep will prove troublesome with a rimless, mesh-covered brackish tank?

[Pufferpunk]

It may.