My usual soil mix is 50% pumice, 25% cocofiber, 25% potting soil (mostly compost, locally produced). During the warmer days of summer I water the plants on your list twice a week, but I keep them outside in the sun. You probably would want to reduce that frequency for indoor plants. Maybe once a week during the season of active growth? It sort of depends on the size of the pot and the amount of light. During late fall and winter you will need to dial back the watering, but I don't think any of your plants will enjoy the soil staying bone dry for any extended period even then. Whatever has worked for your venenifica should work for your poissonii. You might try watering every 2 or 3 weeks during dormant season, given mild temperatures and strong light. You might try asking wherever you got the plants, to see what the grower has to say.
I would caution against relying on a moisture meter (alone) to determine when to water. Most meters are only useful in a relative sense. For example, a reading of "moist" would mean more water in the soil than "wet" but less than "dry". See if you can calibrate the meter against some other measure of soil moisture to come up with an absolute determination. Maybe try lifting the pot, to see how heavy it is, or put a chopstick into the soil and see if it comes out with soil stuck to it, or poke your finger in there. The last two options may not be viable if the pot is small and the plant is relatively root bound (which is probably the case).
You might try measuring how much water the soil absorbs, and compare that to how much it absorbs when it's dry. (Use a small measuring cup to pour on a fixed amount of water, then return the flow through back into the cup and subtract that amount to come up with how much was absorbed.) Over time you will get a better sense of how fast the soil is drying out, these are just some general ways to get at the problem. Another great way to figure out soil moisture is to wait until it's time to repot and then repot when you predict the soil is starting to go dry, but not there yet. Once you get the plant out of its pot, you will have a pretty good visual confirmation of whether your guess was right, and you can adjust the watering accordingly down the road. Do not water right after repotting or potting up your plants, give the roots a few days to heal first.
If you move any of your plants outside, be aware that outdoor sun is much harsher than indoor sun. The difference largely has to do with the way regular window glass filters out much of the UV spectrum, which is invisible to our eyes. So if you're moving an indoor plant outside for the first time, or even a plant you got in the mail (which was probably growing in a greenhouse before), provide a gradual introduction to the sun. Maybe start out in bright shade or filtered light for a couple of weeks, then maybe some morning sun for a couple of weeks, and so on. All of the plants on your list should be able to take full sun if given time to adjust, provided the temperature is not really hot (like desert heat).
One of my favorite references for fat plants is a pair of volumes called "Pachyforms" which you should be able to find online. They both have very nice photos, general information, and helpful blurbs about care of specific plants.
