Possible to Over-Skim?

Is it possible? Yes. Is it likely to happen in normal aquarium environments, no. I would say once you reach a skimmer rated 10x greater than the tank it is going on then maybe you can start to overskim to the point of causing negative effects, but that would be like putting a 1000G rated skimmer on a 100G tank.

Still, I think of skimmers like a work truck, not a sports car. I want it to work with as little maintanence and tweeking as possible and cost a reasonable price. Once you try and get every last bit of performace out of a skimmer the price on the skimmer starts to rise dramatically. Say you can get a decent skimmer that will do a 90% workload without any problems 24/7 for $300 or you can get that uber high end skimmer that will do 97% workload for $1500, is it really work 5x more $ for 7% more effeciency? That is a call each person has to make themselves. Obviously there are many people out there that feel it is worth it to spend the $1500 and I won't say they are wrong doing so.. just have more disposable income than I do for a skimmer.

You'll get more than enough replenishment through foods added and water changes. Most of the trace elements are present in concentrations many, many times greater than seawater to begin with and quite a bit of them don't really drop in concentration. Adding more can build the levels to further toxicity. This is one reason I don't worry too much about trace elements. Beyond that, most trace elements are highly water soluble, meaning that they aren't readily removed by protein skimming or activated carbon usage. Iodine is replaced many times over due to the addition of most commercial foods. I would just add a calcium and alkalinity supplement and be sure to have the appropriate tests available before you begin dosing.

Calcium should remain between 380-450 ppm on average, whereas alkalinity should be anywhere from 7-11 dKH, with higher levels possibly benefitting you more in terms of pH stability (the other plus isn't applicable, since you don't currently have corals).

One parameter shouldn't affect the other tremendously. The only thing that can be affected is the solubility of the other ion. In other words, if too much of one is added or it is added quickly or in low water flow, it will actually cause the other to precipitate. These parameters actually tend to remain steady when adding the opposing ion, as long as you add it carefully. Also, if one concentration is significantly higher than the other--say calcium is at 500 ppm or alkalinity is at 12 dKH, then you'll have a hard time driving up the other parameter, as it will just precipitate.

Despite its popularization, iodine has no proven benefits to any of the animals we keep. In fact, in crustaceans (which it is often recommended for), every effort is made to sequester and purge iodine in the exoskeleton during molting. It is absolutely not necessary for the molting process. Stick to calcium, alkalinity, or if it needs to be raised, magnesium for better conditions. Magnesium is actually important for calcium and alkalinity solubility, as well as coralline algae, which uses it quite substantially. Other supplements have dubious uses, as well, including strontium, molybdenum, etc. None of these have any positive evidence as being beneficial to add in any way. Strontium, particularly, is already present in salt mixes inconcentrations several times greater than that of seawater. I haven't added any of the above supplements (except for calcium, alkalinity, and magnesium) in many years and have had zero issues with snails, crustaceans, etc.

On a separate note, the snail problem could stem from several issues. They are often in bad condition by the time they make it to your tank. There could also be a lack of the appropriate food. Most prefer film algae and a lack of it (or too high of a number of snails) will cause them to starve. Predation is another potential problem, which includes hermit crabs, other snails, etc.

Edit: These may be good articles for you to look at: http://reefkeeping.com/issues/2006-06/rhf/index.php ; http://web.archive.org/web/20021127040526/http://www.advancedaquarist.com/issues/nov2002/chem.htm

The amount of drop in the other parameter should be undetectable. If you are able to measure a drop, then you are adding too much at one time and/or too quickly. The drop in the other parameter means you are precipitating both out of solution. It is impossible to not precipitate some, but the amount that happens should be absolutely tiny as to be undetectable. So tiny in fact that it should be able to redissolve or be neutralized very quickly. It doesn't matter which one you add first, but you need to at least add the alkalinity portion (I recommend adding both slowly) very slowly (if you are using a 2 part buffer), since carbonate is the less soluble of the two ions. Dosing both at the same time, however, can't be done, as it will result in instant precipitation and you end up wasting an entire dose and potentially lowering both of your parameters as the precipitate acts as a seed crystal to drop more out of solution.