I don't know about others, but I have redone my 30 gallon tank 3 times in the last three months. If I would stop buying tanks and trying to find places to set them up, I think my wife would be happy. (wait till she see's the 125 gallon tank I bring home Tuesday)
For the snails, Yeah, I know what you mean, I started with two and end up with 200. They will eventually grow, but being asexual, means they don't need a male or female to have little snails. Each snail can reproduce on it's own! What I have done and I'm fortunate, is I use the small snails in any new tank I set up. They do help even a tiny bit with cycling and cleaning the tank. But for the majority of them I either give them to friends, or ask the LFS if they need any. Sometimes they do sometimes they don't. If they get way too out of hand try getting some Assassin Snails, They eat other snails, don't eat themselves, and reproduce very very slowly.
For your aerator, I have one tank that runs on a sponge filter, does two jobs at once, seems to be working so far, It's my first time using one. So I'm constantly checking on it. My other tank has a sump filter, then is pumped back up through a spray bar. the water dropping from the drain into the filter tank aerates the water before it goes through the filter system.
I haven't used an air stone yet, and I'm not sure on the pros and con's of them. I'll be watching here to see.
I wouldn't think upgrading your filter system would hurt, I would think it would clean more of the water quicker. But I'm not a professional aquariust. Yet.
What I did for my tank changes, is waited until I did a water change, I do a 50% change every week. When I set up a new tank I wait until that day (usually on my day off) to put water in it. In the new tank depending on size, lets use my 20 gallon I'm about to redo for example. I'll add 10 gallons of the water from water change (the old water) and I'll add 10 gallons of fresh water, and I ring the filter I have in my old tank into the new tank. This helps speed up the process of cycling as your water will now have most of the bacteria that you need to cycle. (almost like doing a water change in your normal tank). I let that sit 4 days to a week, meantime checking the ph levels to ensure that it's not fluctuating. Once I feel it's stable I add the fish. One of the reason I let it sit is to help the filter, substrate, plants all get acclimated to the bacteria or vice versa. I add the snails the day I put the water in.
Now I have used the same substrate, plants, and water from my old tank as well. I just vacuum the old sand, into a bucket swish it around to clean it up a bit, then dump the old water out and add the sand to the new tank with a mix of new sand added to both tanks. I break off some of the rooting plants and floating plants and add them in along with the old water. It will be cloudy, but not as much, if you do the sand first, then add the water. I then add the plants, let it clear up, then move the sand and/or plants to the spots I want them. (if i'm going to root them) Otherwise I let them float.(two to three day process) I always use easy growing low light, live plants, they don't take a lot of work, and they help to reduce some of the harmful chemicals in the tank.
As I look it at, both tanks should be almost identical in balance, (not schemes). This way if you have a fish in one tank and think it would look better in the other, you just move them there. I think the only real difference in my tanks are the temperature. Ones a little warmer than the other. That's more of an easy adjustment for fish than a complete chemical balance change.
This may or may not be the correct way to do things, but with my changes, the fish seem to adjust just fine.
Not sure if this what you were looking for but I hope it helps.