Neon Tetras

The first rule of fishkeeping is don't believe a shop till you've researched for yourself. Shop advice is very variable.
Some shops don't train their staff. These shops often tell their staff to make things up if they don't know rather than risk losing a sale.
Some shops are run by old time fish keepers of 'if it ain't broke, don't fix it' mentality, and they train their staff with the same out-dated beliefs.
Most shops are hanging on by a narrow margin. Internet shopping has cut real shops' share of the market. Even if they do know their stuff, customers won't listen to what they don't want to hear and will take their custom elsewhere. So shop owners and their workers tend to not tell the whole truth or they'd go out of business.

MA is one of the better chain stores but even they don't know everything. I was told in one that celestial pearl danios/galaxy rasboras/firework rasboras (all the same fish) grow to 3 inches because they are rasboras and that's how big rasboras get.

You'll read on forums that many, many people have bought neon tetras for a new tank only have them drop dead one by one for no apparent reason. No obvious disease, no marks of being attacked; they look perfectly healthy, just dead. It happens far too often to be co-incidence. The only thing that anyone can come up with is that neons are sensitive to some thing in water that we don't have test kits for. Newly cycled tanks are not mature. They have fluctuating levels of all sorts of things, and it is probably something like this that kills neons. It may well turn out that they need the presence of a particular micro-organism that needs time to grow.

There is a variety of harlequin called purple harlequin. As far as I'm aware it is just a colour morph of the ordinary harlequin so perhaps the golden headed purple one is yet another variety. All of them are Trigonostigma heteromorpha. See notes at the bottom of the page in the link. They would be fine in your tank.

My local MA has the yellow variety of honey gourami and the red ones. Very few shops stock the natural coloured ones as in the photo in the profile on here. I failed to find any which is why I have yellow ones. With both natural and yellow colours, females tend to have a darker stripe running from nose to tail even if it is very faint and broken. A fish without the stripe is likely to be male, but some females don't have it and can be confused with males. But if the fish does have a stripe, it's female. Honey gouramis can be kept as 1m 1f as males aren't as aggressive towards females as some other gourami species.

If you could find one of the dwarf types of cory it would leave you more room for other fish. Look for Corydoras pymaeus (pygmy cory, swims more at mid level that other cories) C. habrosus (salt and pepper cory, not to be confused with the larger peppered cory) and C. hastatus (no common name as far as I'm aware). Google pics so you know what to look for.

Galaxy rasboras are now called celestial pearl danio. MA used to call them firework rasboras, they may still use that name. Be prepared for a shop to use any of those 3 names when you look for them.
I had some of these shortly after they were discovered. They were quite shy fish and you may find they hide in a tank with more boistrous fish like the lemon tetras. I would be inclined to get either galaxys or lemons rather than both. That would give you a bit more space to fit something else down the line. I'm always seeing fish and wishing I could fit them in my tanks 

I think cherry barbs would be OK with that mix.

Red honey gouramis are a bit more difficult to sex. You have to go by fin shape. With all gouramis (except sparkling) males tend to have longer, pointier fins than females. The problem is that you can have young males which still have juvenile (ie female-looking) fins and you can have females with pointier than usual fins.

I will leave that up to the shop and hope they get it right 

The first rule of fishkeeping is don't believe a shop...

Leaving anything "up to the shop" is a recipe for pain and disappointment I'm afraid.

My local MA has a tank full of Honey gouramis..... all with chewed tail fins. Check them carefully!!!

They may grow back (?) or at least you could bargain for a discount 

Yeah.. in Pets at Home the lady said endlers were too small to sex, though luckily there were only males there which was what I wanted!

Yeah all the fish at the time were colourful male adults... Better not to trust a shop unless you've discovered they know their stuff!

They know more than I do at this stage

Believe me, all too often, that is simply not true!

Two weeks on and my harlies still have a tendency to swim up and down one side of the tank at times. Thought they would have settled by now. The cherries on the other hand seem very happy. Zipping around the tank, swimming in in out of my ornamental castle and between the plants