Sorry i am at work doing 100 things at one time of course i forgot to give the link.
Before you add any living creatures to the tank i really recommend to quarantine it first because it can carry the disease. Even the water from the plant if it was in the tank that has ich can carry ich. So everything you put in the tank quarantine . Just buy extra tank that you can use for quarantining.
Here is Oldfishlady water changes recommendations
Check out this link for plants disinfection
Make sure you find out what fish is compatible with bettas and make sure the capacity of the each side enough for the amount of the fish you want to put in there. And of course the more fish you have the more water changes you need to do . Also about compatible fish do not ask the pet store employee. It really better to ask the forum people. Because you might get wrong answer. I already had this experienced once.
A lot of people recommending to have filter double what your tank size is. And make sure the filter not too strong for bettas or you will need to baffle it up.
For fishless cycle. I don't cycle any of my tanks but i do have some tips from the experienced person so if you have more question and don't have help we can ask her for help. Any way that what she explained to me
"" For Fish-less cycling you would, well need an empty tank without fish and you would have to dose Pure Ammonia. That's the hardest part is finding 100% ammonia, many times it's like 10% or has a scent to it instead of regular ammonia. It can't have surfactants in it either. But if you find 100% pure ammonia then it's actually almost easier, well it's quicker to do it that way.
So if you were to do a Fish-less cycle you would dose up to 4ppm of ammonia, so you'd need a dropper of some sort because it's literally drops. But you see the difference there, .25ppm vs. 4ppm so that's going to speed up the cycle for sure. But you don't want to go over 4ppm or 5 because then you risk stalling out your cycle, it's like when you eat too much food and you just want to lay down and sleep all day, the same happens to your BB
So each day, check your ammonia, dose to 4 ppm and if you go to high, just do a small water change to get around the 4 marker. So you do that and the tank should be cycled in about 2-3 weeks. And the good thing about doing Fish-Less cycling is you can handle more fish at one time.
So your colony of bacteria will only grow enough to accommodate the fish you have in there, that's why it's often said to add fish slowly over time, that's so that the BB have time to grow and accommodate for the new fish. But with the fish-less you can fully stock a tank the next day after it's done. Because you BB have grown to accommodate 4ppm's worth of ammonia where as fish-in has only accommodated .25ppm of ammonia, see the difference?
And for a cycle to start you always need a source of ammonia, so either your fish, pure ammonia, sometimes using flakes works too, or you can get some raw shrimp to use. That is explained in the Nitrogen Cycle sticky here too.