Welcome, both.
Glad you're enjoying the fish. I too have harlequin rasboras which are lovely.
LFSs vary hugely in terms of the advice which they give - some can be excellent, some can be very poor, and it often comes down to the individual assistant and even their own particular experience. For that reason, it's definitely worth doing research in advance, so that you know before your visit which fish are possibilities or not
- or deliberately leave the purse/wallet behind so that you don't succumb to temptation and so that your visit is only an investigatory one until you've done the research
.
Neons do indeed tend to fare better in more mature tanks. You haven't mentioned whether the you did a fishless cycle or not -
http://www.thinkfish.co.uk/article/fish-or-fishless-cycling-and-aquarium-maturation and
http://www.thinkfish.co.uk/forums/fishtank-filtration-and-cycling/fishless-cycling-how-to-do-it/ - but even tanks which have been carefully prepared for the fish in this way still tend to need weeks/months to mature more fully before some species of fish are kept in them. [EDIT: Sue replied while I was composing my response - she has posted some very helpful advice to you on this aspect.]
As for the platies, LFSs tend to vary in terms of whether or not they'll take fish back. What I'd recommend you do is not complain as such but just explain that you are new to fishkeeping and hadn't been aware that these fish are livebearers and that this is not a responsibility you wish to have, so would they mind if you returned them - it's possible that you may be expected to buy something else in their place, whether fish or another product, so best to be prepared for this eventuality.
A key issue to establish is whether your water is hard or soft - take a look in your kettle (limestone indicates hard water) and your water company website to find out how soft or hard it is. The reason behind this is because it's best to have fish which are "compatible" with your water. Platies tend to fare better in hard water, so, if your water were soft, then that would be an added reason for returning the platies. Take a look at
http://www.thinkfish.co.uk/fish-profiles which should be help a lot in terms of which fish you like, which are compatible with one another and live in the same temperature range, which suit your water, the minimum tank size required for each, etc.
Have a look at the Community Creator section in the above weblink - it will help you work out whether the other fish you have are enough for your tank or whether you might be able to add any more. If it seems that your tank is already fully stocked with the fish that you have, then an idea might be that, if returning the platies and they wish you to exchange them for something else, that you exchange them for some fish food, dechlorinator, siphon or some other product.
Hope this helps but feel free to fire away with any other questions if this all seems like a minefield!