Sounds like you've been pretty reasonable about it all and its great you've formulated a plan and presented it here!
The trouble is that it sounds like 9 days ago, by taking the fish out, you (without realizing it probably) stopped feeding the two species of beneficial bacteria that were the "engine" of your biofilter. The bacteria would only start to die off a little in the first 2 days, but there could possibly have been significant die-off in 9 days with no ammonia to feed them. On the other hand, bacteria are tough, and a core of the colony may still be hanging on just fine and there may have even been a little ammonia in the basically useless StressZyme.
Its really going to be a lot safer for fish if you first test and verify the state of the biofilter. A biofilter is a miraculous thing, and not having one that is functioning correctly just means that you will have to be a "manual filter" for your fish, laboriously changing water, possibly more than once a day. So, it would be much better to hold on to your plans for the moment and instead make it a priority to get a good test kit so you can find out what's really going on in your tank. The kit needs to be liquid-reagent based. Many of us, me included, like and use the API Freshwater Master Test Kit.
You may have already read other beginner's threads here and seen the recommendations for how to find the correct household ammonia for feeding bacteria when there are no fish. Perhaps you'll be able to find some of this fairly quickly, which might save your bacteria. The problem is that you can't really know without the right test kit and the knowledge of the techniques that you can learn about here.
If all goes well it will still turn out that you have an already, or a quickly cycled filter and you'll be able to get on with stocking your new fish, but now with confidence!
~~waterdrop~~