How toxic any level of total ammonia (TA) might be depends on two factors- the pH and the temperature of the water. This is because, in water, ammonia exists in two forms NH3 aka unionized and NH4 ionized (TA is the sum of NH3 and HH4 present). The former is highly toxic, about 100 times more harmful than NH4. The red line level for NH3 is 0.55 ppm and the level at which it is of little or no concern is 0.22 ppm.
By example a tank at pH 8 and 80 F showing .25 ppm TA would have .0155 ppm of NH3 while a tank at pH 7 and a temperature of 75 would have .0013 ppm. As you can see, neither is a serious worry but there is a clear difference. Now, lets suppose you had a tank with rift lake cichlids which had a pH of 8.5 at 80F. The level of NH3 in that tank would be .0432 ppm and might be harming some fish. Raise the pH by .1 and your .25 ppm of TA now has .0557 ppm of NH3 and you would need to take action to bring it down.
Obviously, one should never ever add ammonia to a tank with living fish or inverts present. Low levels may not do direct harm, what they will do is stress fish to some degree and that in turn opens them up to more serious issues.
Finally, as a matter of personal choice I will never put anything into my tanks which ends with the word zyme.. As a rule one should always look to add the fewest things possible to ones water. There is almost no need to add nitrifying bacteria to a tank more than once in order to get a cycle jump started.
I am not sure what is in the Microbe-Lfit. I do know that Nirtrobacter, which is included, are not the nitriite oxidizers that persist in aquariums, so there is little benefit to having them. What strain of nitrosomonas matters as would the nitrospira. Unless the patent has expired, then Dr. Hovanec and Tetra have control over the common strains found in tanks at this time.
The autotrophic bacteria in tanks do not form spores, so they cannot survive at freezing or at high temperatures, Once you get over 100F you are in danger of killing them, Much above about 105F or so and they die pretty fast. So how they are stored and shipped matter. Moreover, The longer they are in the bottle, the less effective they bcome. After about 6 months you will start to have issues, If you refrigerate them they will remain usefull for about a year. After that, the time it take for them to come out of dormancy and actively function is not much less than starting a fishless cycle without them.
Stop adding all the junk, besides dechlor, to your tank. You do not need it. Also, some dechlors can cause false positives on ammonia tests. Here is what SeaChem states re this:
A salicylate based kit can be used, but with caution. Under the conditions of a salicylate kit the ammonia-Prime complex will be broken down eventually giving a false reading of ammonia (same as with other products like Prime®), so the key with a salicylate kit is to take the reading right away. However, the best solution ;-) is to use our MultiTest™ Ammonia kit... it uses a gas exchange sensor system which is not affected by the presence of Prime® or other similar products. It also has the added advantage that it can detect the more dangerous free ammonia and distinguish it from total ammonia (which is both the free and ionized forms of ammonia (the ionized form is not toxic)).
Most hobby ammonia kits use salicylate reagents. The SeaChem kit is able to test for NH3 ammonia as well as total ammonia.