If you used the mildew resistant silicone, it is in no way sure that it will kill your fish. The stuff that makes the silicone mildew resistant will be a fungicide, and fungicides are frequently used in fish tanks, for example to prevent the formation of fungus on eggs.
The fungicide used in silicone might have a certain toxicity for fish, but if it will be lethal, or have any effect at all, will depend on a variety of factors. For example different fish species might be affected to a different degree. As an example, due to their scaleless skin, catfish tend to be particularly susceptible to any kind of chemical in the water. In addition, your tank size and the amount of silicone you used will matter. If you just sealed a small bulkhead in a 300G tank, there is probably no need to worry. However, if you put a whole rock background in a 55G, using multiple cartridges of Silicone, that might be a more serious problem, because the resulting concentration of fungicide in the water might be quite high.
In the end, nobody can tell you what to do. It is safer to use silicone without fungicide, but if it's a lot of work to rip out your background and remove all traces of silicone - I cannot imagine having to do that in my 240G! - then maybe let the tank stand for a few weeks just with water to leach out fungicide and let the silicone cure. Then try the tank with some guppies. If they survive, you can re-introduce cichlids. I am sure they'll appreciate finding some snack food in the tank when they arrive. :thumb: