If my plants dont need co2, would it kill them if I put any in?
Some of them I listed grow stronger with a bit of co2 apparently.
I was thinking the liquid kind, but will it harm my other plants?
Plants need 17 nutrients. Some are macro-nutrients, others are micro-nutrients. Provided all 17 are present, and provided the light is of sufficient intensity to drive photosynthesis, it will occur. This is how plants grow, as you probably know. As soon as one essential nutrient is missing, photosynthesis will slow. It may stop altogether, depending upon the nutrient, and the light. This is when "problem" algae becomes just that, a problem.
CO2 is carbon, a macro-nutrient. The fish load, and how much they are fed, contribute to the natural CO2. It can also enter from the air at the surface. If you want a tank of healthy fish that happens to have plants in it, then you are working to establish a low-tech or natural method planted tank. You do not add CO2 in such tanks. There will be natural CO2, and usually more than many assume, but if everything is balanced, adding more of CO2 or any other nutrient is more likely to cause problems than improve plant growth--and problems for the fish as wll as plants/algae. There is evidence that diffused CO2 impacts fish. Adding too much of most nutrients also impacts the fish, some more than others.
"Growing stronger" is a subjective term. CO2 is not likely to make any difference, but it all depends upon the light intensity, duration and all other nutrients being available. It is true than in natural or low-tech systems the CO2 is usually the first nutrient to become depleted. However, the fish are what matter.
So-called liquid carbon is usually a derivative of glutaraldehyde. This is the case with API's CO2 Booster and Seachem's Excel. Glutaraldehyde is used in hospitals to disinfect surgical instruments, in embalming fluid, in anti-freeze, in ship ballasts to kill bacteria. It usually kills some plants when used at stated doses, Vallisneria seems especially sensitive. It will kill algae. Substances dissolved in the tank water will enter the fish's bloodstream and internal organs. Anyone who thinks this is not harmful should think again. This simply makes no sense. It is the same as the suggestion that Covid might be killed by injecting oneself with disinfectant. The mind boggles.