There is, either by using a mineral additive for hard water fish, or a calcareous substrate (calcareous means composed of primarily calcium and magnesium which slowly dissolves into the water). But with soft water preferring species in the tank, this is going to make their lives more difficult. With few exceptions, there is no middle road.
The 10g isn't really sufficient space for the neons and guppies (aside from water issues). But having said that, a group of 7-8 neons should be OK if you are regular with water changes, and with floating plants. And the three guppies.
Absolutely. Stress we know causes various health issues for humans, including increased risk of cancer. Fish are no different. Some 95% of all fish disease/health issues are directly caused by stress; the pathogen/parasite has to be present obviously, but it is the stress that allows the fish to succumb. Avoiding or reducing stress is therefore crucial to healthy fish. Stress does without question severely weaken the immune system (in people as in fish for that matter). And inappropriate water parameters (GH, pH and teemp), inappropriate lighting, inappropriate water current, inappropriate tankmates, insufficient numbers of shoaling species, too small a physical space, too little water volume, inappropriate aquascaping--all these factors induce stress. It may help to cite a few excerpts from an article I wrote on stress.
Biology Online defines stress thus:
The sum of the biological reactions to any adverse stimulus—physical, mental or emotional, internal or external—that tends to disturb the organisms homeostasis; should these compensating reactions be inadequate or inappropriate, they may lead to disorders.
Homeostasis is defined as “the tendency of an organism or a cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions.” Physiological homeostasis, or physical equilibrium, is the internal process animals use to maintain their health and life: “the complex chain of internal chemical reactions that keep the pH of its blood steady, its tissues fed, and the immune system functioning” (Muha, 2006).
Four important body functions of homeostasis are closely associated with processes in the gills: gas exchange, hydromineral (osmoregulation) control, acid-base balance [pH] and nitrogenous waste excretion [ammonia]. These processes are possible because of the close proximity of the blood flowing through the gills to the surrounding water, as well as the differences in the chemical composition of these two fluids (Bartelme, 2004). Each species of fish has evolved within a specific environment—and by “environment” in this context we mean everything associated with the water in which the fish lives—and the physiological homeostasis only functions well within that environment. This greater dependence upon their surrounding environment is why fish are more susceptible to stress than many other animals (Wedemeyer, 1996).
Stress is caused by placing a fish in a situation which is beyond its normal level of tolerance (Francis-Floyd, 1990). Stress makes it more difficult for the fish to regulate the normal day-to-day physiological functions—the homeostasis—that are essential to its life. Dr. Cliff Swanson, associate professor at North Carolina’s College of Veterinary Medicine, says that stress creates “a fundamental physiological shift in fish, from energy storage to energy usage—the fight or flight response” (Muha, 2005). The survival of any organism depends upon its ability to keep its internal chemical balance from fluctuating too much. When critical energy is being used to fight stress, it is diverted away from other functions. The fish must then work much harder just to “keep going.” Laura Muha (Muha, 2006) likens this to driving a car up a steep hill: it takes more gas (energy) and effort to maintain the same speed as on level ground (level being the norm for the fish).