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Portulaca is a free-seeding annual, meaning you can cut it down now, it should sprout again in the spring. This article has more information: https://www.gardeningknowhow.com/ornamental/flowers/portulaca/portulaca-plants.htm

Portulaca is a hardy free-seeding annual, so you can cut it down now, it should grow from the seeds in the spring. This article will tell you more: https://www.gardeningknowhow.com/ornamental/flowers/portulaca/portulaca-plants.htm

It actually sounds like your seedlings are suffering from damping off. This article will tell you more about it and how to fix it:
https://www.gardeningknowhow.com/garden-how-to/seeds/what-is-damping-off.htm

You have a mystery! I'm not sure, but I suspect a rabbit may be snapping off your plants, especially if the breaks are clean. If not a rabbit, possibly some other type of wildlife.

Here are some articles that may help:
https://www.gardeningknowhow.com/plant-problems/pests/animals/keep-rabbits-out-of-gardens.htm

When you put more than 1 plant variety in a pot, the important thing is to make sure they have the same water, light, and soil requirements. So you want to look for plants that do well light soil, with not too much water, and good sunlight. Using these requirements, there are literally hundreds of choices. I suggest you visit some plant stores near you, to see what's available. This article might give you some more ideas: https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/exotic-showstoppers-beautiful-succulents.htm

The ASPCA lists Jade plant as toxic to animals and Rainbow Elephant plants as toxic on a level 1, which is oral irritation.

Contact your vet if you animal ingests these plants or any plant they should not be eating.