Yes the ammonia would have helped as that would have acted as though there was fish in the tank. Ammonia is created by food you feed the fish and the waste they produce by eating it, that is now in the water column which then gets sucked into the filter where the good bacteria lives which eats ( breaks down ) the ammonia, the problem is your fish are creating waste by eating but there isnt enough good bacteria in your filter to break it down.
Using ammonia before adding fish is called a fishless cycle, what your doing is a fish in cycle. Its alot of hard work but can still be done, you have to just stick in the with the water changes and as the bacteria builds in your filter your ammonia will drop. Be careful at that stage as your not completely clear as your nitrite may then go up but it may not, just something to keep testing for.
As said above change as much water a possible, by doing this you are basically diluting the ammonia by adding water that hasnt got ammonia in it as long as your tap water hasnt and keep doing it everyday ( I know ) but eventually your filter will be strong enough to break the waste down. If you have a friend with a tank ask them for some of there filter media, sponges, bio stones anything they can give you will speed things up.
Dont add anymore fish until readings are under control. Gouramis arent that hardy so its nots suprising they died, guppies are hardier so lets hope they hang in there. Good luck, any problems or not sure on something just post it on here, I sure someone will help you FAST.