Hey there and welcome to fishkeeping and /r/aquariums.

Unfortunately, your fish store didn't advise you very well, for multiple reasons:

• your tank is too small for the pleco or will be in the long run. I assume it's a bristlenose pleco, not a common pleco; the former will reach 10-12 cm in legth, the latter 30 cm (!) and more. A 15g is too small for either species
• both the molly and cory catfish are species that should be kept in groups of 6+ (this goes for most tropical fish). Keeping them by themselves might work from a theoretical point of view, but you're not doing your fish a favor
• it's generally recommended to cycle a tank before adding fish, not starting a tank and immediately stocking it, too

What you're doing right now is called a fish-in cycle. This is a viable method to cycle, but usually recommended for experienced users. You will have to monitor your water parameters daily, as any significant ammonia / nitrite levels will be harmful for your fish and may poison them. You should do major water changes (80%) whenever ammonia or nitrite are >0, probably daily as well. Expect this to go on for at least 2-3 weeks.

Feeding: You should feed as much as they're eating in the span of a few minutes. Mollys are fine with flakes, corys enjoy algae tabs or anything they can scrounge from the bottom. Plecos are crazy about algae wafers and vegetables (cucumber or zucchini).

Plants: Plant your heart out. There are a lot of plants and most will work well; prepare to add fertilizer once a week, or use root tabs (will last for 6-12 months). Just take a look at what plants your local fish stores have to offer.

Next fish: Your 14g is a rather small tank, barely out of the so-called "nano" range. You will not be able to stock significant numbers of fish! A small group of mollys and a small group of corys will already be enough in terms of bioload and sufficient room for all fish. The pleco will probably have to be rehomed in the long run due to its adult size. Do not get a silver shark! They can reach 30 cm in length and require very large tanks.