Right, I'd scrap the Discus for now ulnless you can garrentee that you can shift them within a few weeks. In a 1 month old system, they will flake and die if kept any longer than a fortnight probibly

Hold fire on them untill their intended tank has been going 6 months before ordering them if there is ANY chance that they will be sitting arround. To do well with discus, you need someone on your staff that knows their stuff on them inside out, upside down and backwards, or people will loose interest quickly. By know their stuff, I don't mean the text book answer you posted above (no offence ment here

). Discus can't read, so you need to understand them and the many ways of keeping them, or they won;t behave as you would expect in a given condition. In many cases, the text books are wrong

Keep modern tank bred stock in soft water in a shop and they will be nothing but endless trouble. Aim for a mildly basic pH and medium hardness, and this is all they will have seen since birth. Idealy, aclimate ALL stock to local tap water,
excluding malawis, or it will flake over in customers tanks, and that is the shop's reputation down the pan.
Congratulations on you new job, you must come across as realy knowing your stuff if they offered it to you without asking
Just remember that you are not keeping fish at home anymore, and that must influence your decisions there

If it won't make you money, don't do it, simple as. Presentation is key. Dress the tanks so they look attractive, but keep it minimal. Avoid hiding places. With 35 tanks in a large store, you cannot practically QT, so all fish in the systems must be on sale, or you won't be able to keep up with demand if the store picks up. With hiding places, you will miss bodies and early infections and raise complaints from customers are a result.
Look at it another way. If you have got your choice of what to stock correct, it will only be there about 2 weeks maximum once on-sale, so it won't have a chance to fully colour up before sale

Where possible, light from the front, as it brings out the colours. Aim for Blue spectrum tubes, preferably with some red, again for the colour, and keep all backgrounds as substrates dark again to showcase the washed-out colours in the best possible way.
Finaly, your supplyer is key. If the stock turns up poor, or goes over fast in your customers tanks, it's time to switch

Well bred fish will easily take the stress of being moved fast without going down with anything IME, and if it doesn't, there is no quality in the stock. You are your customers will benefit from a change
A point overlooked regularly is that of sterilising equipment across tanks. Copper cure overdosed in a central buket, into which all equipment is dipped between uses is an ideal way of doing this, but all staff should be aware that the solution is Carecionogenic (sp?)
Compaired to hobby fish keeping, the retail environemt is a real shock to the system. Back room processes in many "well respected" fish stores is likely to upset many on here. To keep the shops reputation and maintain animal welfare cost-effectively, some things must be done differently to how they are done at home. I can garrentee that at some point you will find a conflict of interest between your hobby and job

Find a way of entering that situation prepaired... In the highly likely event that you enjoy your job though, even at 15, you can make a realy good go at the store and drum up a large an loyal cutomer base with far better than adverage knowlage
Sorry to end on a slightly ominous tone
All the best
Rabbut