When should I remove the new plants forming around the original? Bromeliads can start forming pups at any time, but most start pupping after they bloom. These pups are ready to be separated when they reach about 1/3 to 1/2 the size of the parent plant. If the pup is starting to form roots, that's a good indication that the plant can survive on its own. Pups may be removed by cutting with a sharp knife or clippers as close to the mother plant as possible. The longer you leave the pups on the mother plant the quicker they will reach maturity (taking nourishment from mom). Feel free to trim back the leaves of the parent plant if they start interfering with the growth of a pup. Alternatively, taking the pups a bit smaller will encourage the mother plant to throw more pups sooner. It depends on whether you want a bunch of plants (for bedding or to share with friends) or if you just want a few that will mature quicker. If you are in a colder climate you may want to wait until spring time to remove pups that would otherwise be ready to remove in the winter as pups don't usually root well when it's cold. Alternatively, if you have a number of evenly spaced pups around the mother, you may opt to cut away the mother plant and let the pups form a clump.
What kind of potting mix should I use? Again, this depends on the type of bromeliad, but for most commonly cultivated bromeliads, they like a light, well-draining mix. In San Diego, we have different growing conditions, depending on proximity to the coast. A standard recipe in use by many growers is to make a mix of equal parts of (1) fir or other bark nuggets, (2) perlite and (3) a commercial soil-less potting mix (often called “seed starting mix”). That said, there are as many soil mixes as there are growers, and some plants may do better with more moisture retention in locations more distant from the coast. Many locals have good success with incorporating lava rock or pumice to improve drainage. You want the medium to be able to be moistened easily but drain well. Most epiphytic (growing on trees or other plants) bromeliads do well in this loose, organic medium, but terrestrial bromeliads do better in a mix that retains a bit more moisture. Most terrestrial (growing in the soil) bromeliads do not have leaves that form holding tanks to supply their need for water. These bromeliads include the succulent Dyckias and Hechtias, the grass-like Pitcairnias and the pineapple.
Bonus points if you can name the new Genus [It was Tillandsia.]