Filling my tank

I have a 105ltr tank at the moment I have 4 Honey Gourmi's in it currently and I am planning to expand but am struggling to find the answer to what else can go in with it, I want a britlenose for sure and think that will be fine on it's own, my Gourmi's are all girls so I am assuming this will mean they won't be aggressive as much as males would be I also want to add some discus but can't find out if the tank is to small to them, if it is then fine I am not that attached to the idea of having them I just want a full tank of healthy happy fish and I don't have the option to go bigger than the tank I currently have, any advice of what else you think would go in my tank would be appreciated if you don't have any advice on the discus

What are the tank dimensions (length x width x height)?

An adult pair of Discus need an aquarium that is at least 4 foot long x 2 foot wide x 2 foot high.

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What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).

Depending on what the GH of your water is, will determine what fish you should keep.

Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.

Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.

If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.

If you are in Edinburgh, presumably you are with Scottish Water? It's not straight forward finding hardness on their website, but if you can't find it I can talk you through