Welcome! When I first got into saltwater, my grandparents' bout me a book titled "Saltwater Aquariums for Dummies" and it had a lot of good information in it. I read that front to back as well as surfing online forums/websites for information. I started out with a 38g with hang-on-back filters, skimmers, etc and then eventually made the jump to a sump and a hang-on-back overflow. Ended up with a new job and with that, a 120g with ALL the goodies. My current setup is the 120g with a 40g breeder tank as my sump and my original 38g as a refugium located in my basement. With that being said and having a few years under my belt I can give you these tips:
READ, READ, READ. I still read the forums and learn something new about the hobby everyday. New gear, new practices, etc. Actually KNOWING what you are doing is half the battle. And if you don't know or haven't read it yet, ask on a forum and be weary of your local stores' advice (as well as what you are told online). Some people just don't know but think they do, some people are just trying to make a sale.
Keep up with your maintenance. When I got my new job and my 120g, I just didn't have the time, nor drive, to do water changes. And because of that, I had a saltwater forest instead of a beautiful reef tank. Now that I've got some balance back in my life, and am interested in the hobby again, my tank is looking better everyday. Do your water changes, test your parameters, pay attention to the tank.
Those are my 2 MAIN points. My 38g looked really good before we moved and I had to tear it down and I was pretty much bare bones. I had a heater, the "chinese black box" light, an aqueon powerhead, and my HOB reef octopus skimmer. No $1000 lights, apex controller, automatic water changes. Realize, you don't need all the gadgets to be successful.
To answer your specific questions/points: I'm not familiar with the size of a 30g, but my 38 was very annoying because of how skinny it was. I was very envious of my friend's 40g breeder and if I were to ever downsize, that is what I would go with. I, personally, would start with "dry rock" to avoid any unwanted pests. The 3 times I have set up tanks, I went with "live" sand. This last time, I looked for the play sand that people said works nicely, and saves a lot of money, but could not find it. Clownfish are a great choice for saltwater aquariums. I believe my friend still has my original clownfish, which makes her around 4 years old now. Once I was really into it (almost a year) I had a cleaner shrimp in my 38g and he did fine. I plan on getting one for my 120g once I stock it with more fish. I'm not sure how well 2 would do in a smaller tank. Snails and crabs seems to be one of those topics that are debatable. I had 2 hermits and 4 snails in my 38g. I have 1 hermit and 1 snail in my 120g (due to low fish stock so far). I think it would be wise to add them as you see fit. Just be weary that hermits may attack snails for their shells. I had a a lawnmower blenny in my 38g and he did great at keeping hair algae at bay. When there wasn't algae for him to eat, he readily ate algae pellets. As for corals, do your research and make sure you have the equipment capacity to take care of them. My 38g had a mix of softies and LPS for a decent time and I added SPS months before I had to tear it down. The single black box did perfectly fine over a 38 gallon but I upgraded my lights when I went to the 120g and knew I was REALLY going to get into coral. I had a carpet anemone in my 38g but ended up returning him after he ate my firefish! I, personally, don't want to have to deal with them moving around and being able to capture some of the timid fish I'd like to keep. Anemones are another "do your research" before you buy.
Buying used equipment will save you ALOT. Fish tanks and related equipment don't seem to hold their value. You can get fish tanks, brand new, for $1 per gallon during certain sales. Don't skimp on your heater(s). I bought used ecotech mp10's for about half the cost of them new for my 38g and now am using the cheaper Jebao pumps on my 120g.