undiluted should be fine, Slides answer too.
The tmax, as I recall is a liquid concentrate. If it shows any sign of darkening, then that it a sign that it has oxidized the air in the bottle, rather than just acting on the exposed film silveer halides.
Any dilution of the developer should not be stored for a long period of time without some rather elaborate measures, like glass bottles that are filled to the brim, water boiled to drive off oxygen before being used to dilute the developer syrup, etc. Dilute only for the short term - a day or two worth at once.
Dilute fixer should be ok. Watch for a white/yellow deposit in the dilute and original (liquid I presume) fixer solutions. This, if found, shows that the sulfur has come out of solution - start with fresh product if you find white crud in the bottom.
If not sulfated out, test working strength fixer with a bit of film (like a few inches of leader, snipped before you load the rest of the film onto the daylight reel and put it in the tank). Without putting the film in the developer, and while room lights are on, stick it in the fix solution under test. A clear glass will do for a container. Agitate it in a similar fashion to what you would the tank with film - stir it with a popsicle stick etc once every 30 seconds if that is how often you invert your tank.
Watch the time - the film will go dark and cloudy at first. Then it will gradually clear. Once it has cleared, the best rule is to fix for twice as long as it takes the sample to clear. It will take longer to clear as the solution gets used .
After it takes twice as long to clear as it did when it was fresh, I pack it up and ship it off to my city's dump's hazardous material depot. They are pleased to take it. Camera stores with labs on site likely will take the used solution - it has silver in it, and with the right gear there is money in getting the silver out of the solution of spent fixer.
As far as slides go, I presume you mean commercially processed E-6 slides.
Exposure and time/temperature of the first developer set the density. Slides are finicky. You need to pick low contrast scenes, or use graduated denisty filters to overcome the limits of slides lesser dynamic range. Where they lack dynamic range of differnt light levels they excell at colur saturation.
C-41 negatives on the other hand, deal with light range better, and colour gamut less well than slides. I think of most slides as handling about 5 stop of dynamic range (lightest to darkest) at the outside, while C-41 negs can handle 7-8 stops in some films. B&W has a wider dynamic range yet again, if developed appropriately.
I have shot slides well, and then screwed them up because I am crazy enough to try doing my own slide processing in my darkroom efforts. If you are shooting slides and having a commercial operation process them, then the density should be spot on from roll to roll.
