Its kind of hard to guess whats wrong when there are no specific symptoms. Have you noticed any stunting, yellowing, brown spots, pinholes etc?
Just general slow growth in my tank usually means Ive let the floating plants take over and there isn't enough light, the bulbs need changing or I've under dosed nitrates again. Could you give your most recent test results? Maybe something will jump out.
Shops have a high turnover of animals and corals so you never know what hitchhikers they have brought in so I doubt its worth the risk. Thats assuming you find a shop that would be willing to help you. A large part of fishkeeping is about patience and research. The set up is all part of the fun and will give you time to get to grips with maintenance. I expect you will have your hands full anyway with learning to keep such a small tank stable.
This forum is almost exclusively freshwater so you will need to do more research elsewhere but here are a few pointers:
Be wary of following the tank instructions without doing your own research. While it does tally with that I know about marine I'm not an expert. You may find that the manufacturers way of doing things is not always the best way. They are suggesting that you use live rock, sand and water from another tank that you already have running when they suggest using 'mature water'. If you don't already have a marine tank you most likely cant get those things and need to cycle the tank yourself. They have instructions for that on page 8. 'Prepared seawater' will be RO filtered water from an aquatics shop and marine salt and the next page is actually a guide in how to make/use it.
Re: Are peace lily flowers poisonous to goldfish?
Its probably a mixture of issues but the tank size and possible water quality ones can be fixed. You cant do anything about poisoning.
To give them the best chance short term keep up partial water changes of about 50% a day and buy them a large foodsafe box to live in. Asda and Staples do good ones that are 100 liters+ and they don't cost a lot. Fish do not do well with sudden changes to their water so you will need to bag them up and acclimatise them to the new 'tank' by floating the bag in the box and slowly adding more new water to the bag over the space of an hour. The you can tip the bag and let them swim free.
I did a little research on this myself and but decided it looked like a bit of a pain. The tank size issue is gotten around by having multiple bottles which can also be topped up at different times to give a more consistent output. Even then you need to really keep on top of it to make sure the CO2 levels dont fluctuate too much and cause algae blooms. At night you probably need to run an airstone.