Hi Luke, Didn't know the critter related question is why I didn't respond earlier. Scaleless Kuhli and Clowns yes but not never axolotls.
Regarding bagged dirt and the setup you are talking about that I can mumble about a little.
ix is the bagged 'dirt' product I use.
(potting mix) contains soils and a large portion of organic material (55-65% by volume).
Listed as containing Sphagnum peat moss, composted bark fines, leaves, twigs, wood chips, etc. and "pasteurized poultry litter" (cooked chicken crap).
The chicken crap is cooked to eliminate the bacteria it's commonly known to contain. Using it I've never seen 'level' spikes to be concerned with. Several tanks went months without ever testing above zero or 5ppm on nitrate and phosphate but plant growth was outstanding.
0.10-0.05-0.05 is one of the lowest NPK ratios you will find on a bagged product and part of the reason the initial spikes reported by many using other stuff are reported at a minimum using MGOCPM.
Get the right bag and have zero point zero added chemical ferts to be concerned with. If you want to avoid ammonia spikes and other water parameter hiccups I suggest avoiding soils with manure in high percentage. Plant heavily with what you have. Root feeders right from the start, also I like to use floaters (you mentioned some) because they aid in controlling light and the filter feeding roots from floaters minimize anything left for algae to feed on. Stock lightly and control the light energy betting you no issues but can't guarantee anything.
Using seeded media (which you have) setting up over a dozen of these type tanking systems I've only tested positive for ammonia and nitrite on tested values twice. Both times in less than 48hrs it was over.