Mixing grow soil brands

Should be fine, most bagged soils and soilless mixes are about all the same shit. Peat and its many forms, compost. worm castings, coir, perlite, vermiculite exct. its all interchangeable. Some small bits are different but doubt it will be crazy shock to anything. even if you use pure soilless like peat and perlite and top dress with a pure soil like happy frog it will work just fine.

If it's mixed together it's fine. You don't want layers of your substrate with drastically different drainage properties. that leads to potentially funky drying patterns and root zone issues. A little plug of coco in something different when you pot up some seedlings is not a big deal, but not a best practice either. if they are both soils and roughly same water capacity/aeration, then you can get away with it. If you can mix anyn leftovers together that's the better way to go as it reduces any differences. one question -- why do you need to add 2L to the top of a pot? this means you didn't fill it properly to start if there is that much excess room. don't do that. just use a smaller pot and pot up when it needs a bigger space for roots. Sometimes you bury a plant a bit deeper if it stretches, but that's not an ideal behaviour to repeat in a standard way -- adding 2L will raise 'ground' level relateve to stem. the stem that was one exposed will eventually sprout roots and adapt, but before then all you have is slightly buried wet stem 24/7, which is a potential pathogen risk you want to minimize not maximize. You only do it when necessary not as part of standard procedure.

If you want to add 2 litres of BioBizz on top of the 8 litres of the Plagron, it will be just fine. If you were planting from new, mixing the soils together is better than having soil in layers, but what you are wanting to do is basically top dressing your pot, adding the layer of different soil on top is ok, the plant will not suffer in any way.

They are similar products so the amounts your mixing will be fine.