The test strips are Tetra 1 in 6 (just checked) and present the colour codes for results in °dH which I presume to be degrees of hardness?
Yes, dGH (or dH sometimes). So that means moderately soft/hard at 8 dGH.
I can't honestly say I fully understand all of your reply. I now wish I'd taken chemistry at school! The reason I thought the wood may have contributed to the change was because I noticed bubbles rising to the water surface from it.
Don't worry, there is just a minimal amount of chemistry an aquarist must understand so he/she doesn't do something harmful trying to adjust parameters. You asked about the pH lowering in the aquarium, and the organics breakinbg down is one cause, but here the GH and KH are moderate so this is going to be minimal. There is another thing though, and that is that your water authority might add something to raise just the pH; soft and acidic water areas often do this to prevent corrosion of pipes and appliances due to acidic pH levels. You could check with your authority and see if they do this, as frequently whatever they add is not permanent so the pH will again tend to lower some in the aquarium. But whatever the reason here, you have no issues with an acidic pH so long as you stay with soft water fish (no livebearers and similar fish that must have harder water than you have).
Bubbles from the wood is most likely just air inside; eventually the wood will be waterlogged and this will cease.
Do you think my water parameters are ok to go ahead and start stocking with the hardy fish; Rummynose Tetras. When I do, should I put in the 12 or will it be safer to put them in in smaller groups to avoid an ammonia spike?
The Aqua Pure balls I assume from looking them up are a bacterial supplement to aid in cycling. You also have live plants, and these take up ammonia and quite a bit, so you should be OK for fish. Fast-growing plants, and floating plants are best for this and in fact all but mandatory with discus (and rummy's for that matter, neither like overhead light), and once established it is almost impossible to add too many fish with respect to ammonia; floating plants are "ammonia sinks."
Rummynose tetras are actually not hardy, quite the opposite. And shoaling species like the tetras should always be added as the entire intended number whenever this is possible (i.e., the store has the number you want). The more there are, the easier they will settle in with less stress. Some species may have hierarchies and the entire intended number is even more critical but all shoaling fish should be added as the entire group. You will not have ammonia issue with live plants, plus there is the tank size (more water reduces this risk).
On the rummynose, I would get a few more, here 20-21. I had this number in my 70g (4-foot) tank and it worked very well. This is a species that tends to stay in a shoal more than most other tetras, and it will always fare better in larger numbers.
If this were me, I might consider putting the discus in first. Again, being a shoaling species, add all of the intended number together. I think I already suggested seven as a better number. I know they are probably expensive fish, but this really does make a difference to their health, and you are more likely to avoid any hierarchical problems. The discus can settle in to a fish-empty space, and will be less stressed than if they are introduced to a tank with fish swimming all over the place. Rummys are active swimmers, but not to the extent it will bother the discus. And they can tolerate the needed warmth of discus. So can the false/green neon tetra, a close relative of the cardinal but better in warmer tanks even than cardinals. Angelfish would find them food, but discus should not unless they are full grown and it is better not to acquire full grown discus. I believe the discus authorities suggest 3-inch fish as best.
And finally, should I wait until the wood has stopped showing fungus/bacteria? Obviously, I am excited to be almost at the stocking stage but I don't want my eagerness to be detrimental to any fish.
I don't know what to suggest concerning this wood. Azalea is in my mind as possibly an issue for fungus, and it would be a shame to add a group of discus and have them die overnight. The rummynose would be better given this, thinking of the expense. I wonder if other members have had fungus issues with Azalea?