now for the first question, they say not to mix the stratum, but I did anyways, so I think it's fine, so long you have the potting soil down first. The stratum is quite light weight, but it makes planting easy. The canister should last you anywhere from one to two weeks depending how you fill the bell diffuser-either one chamber full or two. With the provided chamber the one bubble per second does not apply, just fill the chamber and then let it set until it's all dissolved. Anyways back to your second question, I haven't had luck finding any of these chamber at my local fish store nor do the replacement canisters at the sports stores here have them, plus you really can't use the any other co2 canisters as they have to have the correct threading on them.
Hope that answers your questions, and good luck in setting up your aquarium.