Hey PsyDuckmonkey,
I live in a colder environment without much humidity as well. The sensor in one of my grow tents reads 21C and 41% right now, but thats only because I just watered everything (it is usually around 30%). I've got a bunch of plants currently growing in my place, including Echinopsis Pachanoi, Echinopsis Bridgesii, Echinopsis Peruvianus, Mytragyna Speciosa, Mimosa Tenuiflora, Phalaris Aquatica, Phalaris Arundinacea, Acacia Acuminata, and Banisteriopsis Caapi. And some other potentially interesting plants such as Silene Capensis, Plectranthus Scutellarioides, Fittonia Albivenis, Calea Zacatechichi, Delosperma Nubigenum. And then there are some that I used to grow, like Sceletium Tortuosum, Phalaris Brachystachys, Cannabis, Leonotis Nepetifolia and Heimia Salicifolia. And thats not including the other houseplants and cacti I collect.
Yes, my place looks like a jungle and watering takes a long time.
Anyways, most of my plants started out in a small 2'x2'x4' tent with a 600watt led grow light and a little grow fan. Everything grew great in there, with the exception of Delosperma Nubigenum, Silene Capensis, and Banisteriopsis Caapi, which all seemed to dislike the direct light and quickly had wilted leaves. They are now outside of the tent (30-60cm maybe?) and getting indirect light which shows through the open doors on my tents. I have one Banisteriopsis Caapi clone by a north facing window getting indirect light, and right next to it is my Mimosa Tenuiflora. The trick to making these two happy, as well as the Mytragyna Speciosa, was to have a humidifier by them 24hrs a day running on low. Slowly, while keeping an eye on the leaves to see if they wilt or fall off, I would run the humidifier less and less. And if the leaves looked hurt, I ran the humidifier longer. Now I don't even have one in my place at all. And not sure if it mattered or not, but I should also probably mention that the Mimosa Tenuiflora was seed grown, and the Banisteriopsis Caapi and Mytragyna Speciosa were small clones.
Sorry for the wall of text, just thought that you might be interested in that information as well. Or maybe there is someone else who stumbles on this thread and is curious about growing any of these plants indoors.
But, on to your question. While living with limited space, low humidity, and a steady temperature of about 20C, I would grow the following:
1. Acacia Acuminata: Slow growing, but does well under the 600watt and doesn't seem very picky. Mine are still young (~1 years old, so take what I say with a grain of salt here), but they seem to be growing well. Plus, you don't have to rip up roots when they are ready like with Mimosa Tenuiflora. I'm hoping that there is someone else out there with some more experience with this one and can comment
Other plants to consider (Which may interest you or others in a similar situation):
2. TBM (Trichocereus Bridgesii Monstrose): They are pretty small and grow excellent under a small light like the 600watt led. Very easy to clone too - Just cut off a ball and wait until roots show, then plant it.
3. Banisteriopsis Caapi: Very easy to grow even without a light. Just plop it by a window and keep watering the thirsty fella. Also very easy to clone - Just cut off a section and put it into a glass of water until roots show, then plant it.
Having never grown Psychotria Viridis, I cannot really comment on that one. But, Mimosa tenuiflora, while growing well in our similar environment once acclimated, needs a lot of space. Mostly it requires a large pot for roots to spread out and do their thing, which is a pain to move when it gets older. Moving my large ceramic pot around while getting poked with thorn protected branches isn't fun. It is a beautiful plant though