Stocking Ideas For 105l With Harlequin Rasboras

Your water sounds almost identical to mine, and the fish you have are very much ones "on my radar" (or ones I'd have if I had a tank your size ie bigger than mine). 
Cardinal tetras tend to have a reputation for doing better than neon tetras.  I've had some for a few years but, admittedly, they haven't done as well as harlequin rasboras and espei rasboras.  They do get on well with the espei rasboras and did with the harlequin rasboras in their short overlap.
Micro-sized rasbora (eg neon green or brigittae or chili) will have similar problems to your embers, and neon greens didn't fare well with me, so I'd omit those.
Another type of tetra might be worth considering eg lemon, glowlight, black neon. X-rays did well in my tank, and alongside harlequin rasboras, but were a a bit boisterous ie males sparring.
Even a peaceful barb (ie cherry or fiveband) probably wouldn't fare well with the shrimp or gourami, but someone else may have a better view on this.
Cories (eg panda, julii), I wonder..?

I agree re lemon tetras; I'd love to have them myself but unfortunately my tank is too small - yours would be fine, though.
I have my tank set at 25'C too, with the cardinals in it which seems to be within their range:
https://www.thinkfish.co.uk/fish/cardinal-tetra.html
https://www.seriouslyfish.com/species/paracheirodon-axelrodi/
You're right re cories - they wouldn't do well on gravel, and I forgot you had that (apologies).

Keep us posted re what you opt for. Photos also very welcome of your tank once the newbies settle in.   

I finally went out and bought some fish today.

I got 8 lemon tetras. I was also considering glowlight tetra, dwarf pencil fish, threadfin rainbow fish and diamond tetra, but I couldn't find these in my LFS.

They don't look yellow at all yet (apart from the fins), but hopefully they will brighten up once they are settled in my aquarium!

Here is a photo of some of the lemon tetras. They seem to be quite hardy fish and spend most of the time swimming in a shoal with the harlequins in the centre of the tank. They are still not looking that yellow yet though, not sure if they are meant to only be a pale yellow, or if they get more yellow once they are fully grown.   

I had a bit of bad luck with my aquarium in the last month. Firstly the honey gourami died, it is the third time I have lost a honey gourami within a few weeks of purchase. I tried to keep one 2 years ago and it died, and thought they might do better in pairs so tried again a few months ago but exactly the same thing happened. They seem to gradually lose their colour and go lethargic. I wonder if they are infected with a virus like dwarf gourami are, or are just genetically weak. I think I will give up on gourami now.

Last weekend I found a leak at the bottom of the aquarium. Managed to get a replacement aquarium delivered later that day. I also decided to use it as an opportunity to replace the blue gravel with black sand. All the fish and shrimp survived being caught and put into a holding tank for an hour (I was really worried I would lose 1 or 2 to the stress). Water parameters are also very stable in the new aquarium. The black sand looks great, but not sure if it is suitable for corys as the grains seem quite large.

 

It would be nice to try keeping cories again! The sand is very smooth but the grains are quite large. Not sure if the grain size is an issue too.