What are your water parameters?
Many of the northern South American Geos (Venezuela/Colombia) need pH of 6 or below, very soft water .
Those from mid Amazonia 7 ish, fairly soft water.
Geo braziliensus group are very variable to hard, high pH water
Those from west of the Andies tolerate pH values lightly above 7, and slightly alkaline water,
Geo crassilabrus from Panama higher pH, into pH 8 and above.
Some Gymnogeophagines, are very adaptable to diverse water Parameters.

 

Tap Water Immediately
pH 7.4-7.6
KH 4°
GH 4°

Tap Water after resting for 12HRS
pH 7.4-7.6
KH 4°
GH 4°

I wouldn't be against installing a RODI system.

 

Your water seems perfect for all, except the Rio Negro/Orinoco northern S American low pH, acidic , black water species.

 

4 g. sp alto (gold dust) would work; prefer 3 female and a male. they are very flexible and prefer cooler temps - 72-78.

 

Stocking with a community of so-called ‘cold water’ Cichlids originating from the southern regions of South America, would make for a nice aquarium.
Gymnogeophagus species such as G. balzanii or G. meridionalis are smaller growing Earth Eaters that would make for a fairly peaceful, really nice looking group.
Add a couple Australheros (‘Chanchito’) species Cichlids to go with that Earth Eater group, and you’ll have the Cichlid thing covered pretty well. Dithers (if desired) would need to be cool water tolerant species, such as Jordanella Floridae, American Flag Fish, Platies or Barbs…..
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NOTE: Cold Water tanks stocked with Cichlid species originating from cooler weather regions, require an annual ‘rest period’ of lower tank temps to look and be at their best. If your home dips down into the 60’s (or even lower!) during winter, simply leaving the aquarium heater off during that time will provide everyone in the tank with the cooler temps they need for the required, annual rest period.

 

Would Sveni or Red Head Tapajos be suitable, or do those fall into the Rio Negro/Orinoco category of northern South American, low-pH, acidic, blackwater species? If so, what are good tankmates? Do EBAs work?

I have 90 G available and 150 G ready in a couple of months. The geos you recommended are beautiful, but they don't stand out to me.

 

Okay then……
A 1M/4-5F group of Geophagus pyrocephalus, Red Head Tapajos will do just fine in your water, dechlorinated out of the tap, ‘as is’. A bit smaller growing in size than some other Earth Eater species, the Red Head Tapajos may be the most aggressive species of the lot.
So, more females to spread out that high-level of aggression shown by the males of this species.
Beyond that, further stocking recommendations really will depend on what aquarium these Cichlids will be kept in, long-term.
Be great if you plan on going with the 150 gallon tank (72 inches X 18 inches bottom dimensions?). But, if this will be long-term for the 90 gallon aquarium, that is do-able as well…..

 

Gonna be in 90 Gallon for a couple of months until 150 is ready. What could be potential further stocking for either?

 

For the 90 gallon, a single EBA would be pretty nice to go with that Earth Eater group. For the 150, a single Green Terror would be a good addition (ensure the Earth Eaters get some size to them first, before stocking with the quite predatory GT. Or, if you can find one of these instead of the more common Green Terror, a much calmer, more peaceful Aequidens diadema, Diamond Acara, would be a good choice. Otherwise, Mesoheros ornatus. M. gephyrus or even better, a M. atromaculatum would be very nice to stock with as an individual, larger growing, sort of ‘center piece’ Cichlid for the tank.