Your cactus looks quite healthy.
The location sounds good, with lots of sun through the window (indoor sun is not direct though as regular window glass filters out most of the UV). The southern orientation will be ideal in the winter when the sun is lower in the sky, provided there are not too many trees or structures outside to block the light. These plants enjoy lots and lots of sun and it's impossible for you to give too much inside. Just be careful if and when you choose to take the plant outside when the weather is good, you need to provide a gradual stepwise accommodation to the sun, starting with bright shade.
If you want to repot your cactus, I would recommend a soil mix with more rock. I like to use 50% pumice for my cacti, but whatever you have available (eg. perlite, lava rock) should work fine too. The problem with an all-organic mix is the risk of rot, which increases when the soil remains wet in between waterings. A nice fast draining mix helps a lot in avoiding this. To figure out what size pot will work, you need to have some idea of the size and shape of the root mass. These plants grow in cracks in the rocks in nature and do not demand a lot of space in cultivation. Ideally you should look for a pot that closely matches the size of the existing roots, plus a little extra space. You can certainly trim the roots but ideally find a pot with a good fit. Avoid watering for a week or two after repotting to be on the safe side, longer if you do serious root pruning.
The little plants will fit just fine into pots about 1.5 to 2 times their width, if you want to separate them. I like to use pots that are wider than deep, but not with dish-like proportions. Again, these plants grow in cracks in the rock so they do not need lots of space.
Watering will depend on the exact conditions but you want to soak the soil until water comes out the hole in the bottom, then wait however long it takes for the soil to dry out at depth. Poke your finger or a chopstick or a moisture meter in there if you are not sure, and be aware that the soil at the surface will dry out faster and not be informative about what's happening deeper in. Once the soil is properly dry, water and repeat the cycle.
