Peat is organic matter. Sphagnum peat isn't made up of the kind of material you find in the 'peat' part of hydric soils. Sphagnum peat is made up of the leaves and stems of a moss. Natural peat found with submerged aquatic vegetation (SAV) is the roots of aquatic plants, the upper parts are either washed away when the seasons change or fully mineralized in the fluff layer before becoming part of the sediments. If the peat is made up of more leaves and stems the conditions are almost always too acidic for SAV's. That's when you get a swamp with everything either floating or emergent, essencially just plants that can access the atmosphere.
Mineralized soils have little organic matter, almost everything is completely decomposed. If you want to use soil in your tank avoid potting soils and anything that has humus added. Mineralized soils have more nutrients in them than peat soils, basically because the nutrients in peat are 'concentrated’ after being decomposed and removed from the connective and structural tissues of the plants. Mineralized topsoils are great for long term aquaria.
Humic soils have peat and peat precursors which will decompose rapidly when submerged. All it takes to let bacteria finish the decomposition of peat is oxygen. Something that is in abundance the first day of a submerged peat substrate and increasing again as the plants put out roots and begin pumping oxygen to these roots. These are the soils that produce the 'royal messes' in tanks, avoid them.
Can you use peat? Yes, in small amounts it works fine, best under a 'cap' layer that prevents contact with the main water body of the tank. If your peat contains sulpher, something peat does contain just like soft coal, there is a potential for H2S formation. However, if you limit the amount of peat, the amount of H2S formed won't be toxic and probably will be cycled again as the tank ages.