Though this thing isn't perfect, try using this online aquarium calculator: http://www.aqadvisor.com
You insert your tank size/dimensions, the filtration, then the amount of fauna you have and it'll tell any warnings it thinks (ie: shoaling fish not having enough fish in it, aggressive fish, or in your case possible over population)
For the other things you mentioned, I'll give my 2 cents from what I've experienced and from what I learned from these boards:
- Don't use PH down
- Don't worry about your PH being 7.3 I rather it 7.3 then trying to artificially make it go down which will stress out your fish and not be a constant
- Don't consider getting PH for the main reason to lower PH (the main reason would be for plants)
- Do consider lowering the PH with natural buffers such as driftwood, peat, or substrate that has a peat in it that will lower the PH.
- Do consider c02 if you aren't satisfied with your plant growth or if you want to try something new
If your plants are doing fine and you're happy with them, then don't bother with c02. The hardest part of this hobby is to leave things alone and appreciate what you have - if you're just bored and want to spend money to make your tank different, you could just try adding some cheap/free driftwood or change the aquascape a bit. Adding c02 is expensive (and you complained about the cost of substrate which is cheaper than a whole injected c02 setup and will help plants grow a lot) so I wouldn't worry about doing it if you're happy with your tank and plants.
For your snail problem, try not overfeeding. What's overfeeding? Feeding every single day, feeding so much that it hits the ground (though I guess you have corys so you may intentionally do it). But snail population will get out of hand if you're overfeeding. I'd suggest feeding 2-3 days then skipping 1 day. They could go many days (some say weeks) without food but they're our pets so we feel obligated to feed them all the time, but just lay off a bit and it'll be healthy for them and your snail population will hopefully not explode too much.