I would check your soil ph,,,,,I have saw those little rust spots many times on different plants ,,,usually a cal,,deficiency,,since you do not know the soil type,,I would check there first,,BUY a kit there cheap ,,if you do not want a test kit ole fashion way is small amount of the same dirt in little shot glass,,pour small amount of vinegar on it if it fizzles it is high in alk,,,do another sample mix small portion of water on dry dirt add baking soda,,if it fizzels with baking soda ph is high either way if this is the case you need to mix a teaspoon of lime to a gallon of water,shake it well and give them each a taste ,,it will help correct your soil,,,CHECK THE PH before doing anything,,less is more so do not start trying a bunch of different things,,you can make it worse ,,investigate and isolate the problem and then make the right call one time,,,Will say this Lime is not a nutrient ,,,But does have many things in it that will help a soil and the micro organism thrives and it will help your plant uptake more of the nutrients it needs,,,,,,if they thrive the quality of the soil is then improved,,same reason to feed a little molasses,,,not as a nutrient but it helps the things living in your soil thrive,,if you have poor soil,,the plant can only do so much,,, in any said medium,,, the better the medium or dirt,,, the better the plant can thrive,,,,Also Pot size does and doesn't matter I have grown 24 inch healthy plants in a beer cup to prove this,,they will grow massive roots which makes them thrive greater after transplant,,,The amount of water or nutrients,,is what it will come down to as more exposed roots in a smaller pot are more likely to get burn ,,in my XP,,,But over watering will make your soil very heavy and can suffocate roots as well,,take a few minutes and look over your plant very close ,,look for any sign of bugs ,,eaten portions of leave,,tiny webs,,and so fourth,,The people here will try to help give you good advice and help,,,do not rush to try 100 different things,,,do the 1 right thing,,KISS keep it simple stupid