My girl had chronic kidney disease with mild HCM when her vet started her on fluids. That led to a CHF episode, and at the same time got diagnosed with either SCL (small cell lymphoma or extreme IBD). We couldn't biopsy to diagnose because of her heart - she also had a heart block. I did start her on budesonide instead of pred, which her heart couldn't take. The budesonide seemed to be OK for her. She was also on Leukeran.
Turns out the CHF at that point reversed itself once fluids were stopped and heart meds started. She was not on lasix at that time. I was seeing an internal medicine vet at the time to manage all of her conditions, and also consulting with a cardiologist. Fast forward several months and her kidney values got worse, IM vet wanted me to add just a little bit of fluids every other day. However, that was a mistake. Led to the second CHF, and this time we did have to add lasix, but just a little bit.
Bottom line, the heart trumps kidneys. Not sure what type of specialist you are seeing, but I would recommend going to a cardiologist. Depending on what's wrong with the heart, there are medicines that can help. I would add as much water to your kitties food as Leo can tolerate. An IM vet can help balance multiple conditions and meds, but get the cardiologist to help.