How long has the tank been running with fish? I reset my 70g recently and filled it, the pH of our tap water is 7.0-7.2 but less than 1 dGH and KH, so no buffering to speak of. It is now 2.5 weeks and only yesterday did the pH lower to 6.6 (having been 6.8-7.0 previously). So it takes some period of time, depending upon the hardness and other factors. In the past I have found tanks tend to settle in 2-3 months, but again I have near zero hardness and you have some so that will slow the lowering.
On the fish. Angels are shoaling fish--I just revised the profile day before yesterday, so have a look, I took considerable care to include details on compatible fish, etc. You will probably have the Scalare Angelfish, Pterophyllum scalare; it is the most common, widely available, and tank raised unless you have an importer. The other two species, P. altum [a real beauty] and P. leopoldi will be wild caught and very rare, although Altums are showing up here more frequently, all direct imports of course. Shoaling--that means a group, 5 would be fine in a 66g (250L) tank. Never, never alone, these are social fish and should only be out of a group when spawning. The aggressiveness is covered in the profile, note esp the bit about getting all five at one time to avoid problems with dominant fish.
Gourami are very similar, note the profile info. I do not recommend combining gourami and angels, so in my view one or the other. Rainbows the same, either make them the "centre" fish, or the angels, or the gourami. Note that Rainbows prefer basic harder water, the only fish in your list that does, so given the water parameters that you have I would tend to stay away from Rainbows in this tank.
Tetra are fine, just get them sooner rather than later as small tetra can become snacks for angels [if you go with them]. Other tetras in the Rosy Tetra clade [Hyphessobrycon species, several are in the profiles under characins, use the scientific ordering and they'll appear consecutively in the list] are good with either angels or gourami. And corys, fine. Groups of five per species is ideal.
You have room for some other "odd" bottom fish, like one of this or that. Whiptail cats, a smaller-sized pleco (some max at 4-5 inches; the common pleco is 18 inches, not good in a 66g), etc. Corys manage fine with any of these, and I always like some bottom interest. Hatchetfish for the top, but not with gourami; angels are OK with the larger hatchet species like the Silver Hatchet, although in Carnegiella the marbles would probably do OK, Carnegiella strigata.
On the plants, the Water Sprite may be Ceratopteris thalictroides which is finely pinnate compared to the more common (and lovely floating plant) Ceratopteris cornuta. And floating plants are a good idea with all forest fish including angels and gourami.
Byron.