First of all, it is difficult to imagine a higher critical mass of ammonia/ammonium/nitrite than you've already experienced from dumping a bag full of terrestrial soil into your tank and exposing it to water. The fact that all your parameters are headed in the right direction is a huge testament to your healthy plant growth. If you're really worried about bio-load, the solution is to continue what you've been doing which is adding more plants. Hoping that you have enough beneficial bacteria to absorb fish poop is like hoping you have enough ants at a picnic. Just eat the food.So they do not die due to ammonium/nitrite toxicity? I guess a high fish load will increase ammonia/ammonium/nitrite levels in the tank dependent upon the amount of fish and how much you feed them. To then be sure your tank can handle that amount of ammonia/ammonium/nitrite it is ok to have a back up system (filter with beneficial bacteria) in case your plants and your tank can not handle the ammonia/ammonium/nitrite levels.
Second of all, as @dwalstad has posted several times, your unusually high nitrite levels were likely not the result of nitrification (beneficial bacteria) but were probably the result of nitrate respiration, a process that was set in motion while the soil was exposed to air.
Third and lastly, be careful what you wish for. The less able your plants are to compete with the beneficial bacteria already in your tank for food, the more you will periodically have to change the water as nitrification levels rise. Plant uptake of ammonia/ammonium results in plant protein. Bacteria just produce a different form of nitrogen.
It's probably too early to expect toxic gases to form in your soil. I would consider poking around now rather than later as a preventative measure.From one thing to another, I now have bubbles in the soil of both tanks. I did try to sm,ell while I poke the bottom layer, could not smell sulfide/anything. What does that imply? What should the poking be good for, is it necessary?