Question about planted aquarium and fishless cycling

Well, I am a little sad. I woke up this morning to find my beautiful yellow male guppy and a pretty little yellow and red guppy dead. PH is back to 8.2, ammonia is 0.25, nitrite 0, & nitrate is 40. Is the ammonia high enough to have caused their death?
I was already thinking about a quarantine tank for when I am ready to add fish.
To answer you question about other fish, I was thinking 6 neon tetras, 4 panda Cory, and maybe 2 honey gourami. The kids really want me to get some glofish as well. But at this point I just need my current fish to survive. Thinking a water exchange is in order. I do believe 2 of my female guppies that are left are pregnant. I doubt I have enough plants for them to survive at this point.