Your first question is impossible to answer because different Semp. varieties grow at different speeds. Some have large rosettes and some small. One plant may only grow a single extra rosette in a year and another may increase in size by 10 times. The best way to help the plant to increase in size is to give it ideal conditions. I would prefer to leave the plant whole until the rosettes are a good size. Don't be tempted to remove them when they are too small. To answer the last question - yes Sempervivums will cross pollinate with enthusiasm but the majority of seedlings will be very uninteresting. Those who raise new plants know that you have to grow a lot of seedlings to find a new plant that's special enough to keep. When there are so many quality named plants available it would be a pity to end up with a display of mediocre plants. I think that the space around the plants is important. A few really good ones in a gravel bed can look better than a lot crammed in together. You can then see the special personality of each plant. I'd be tempted to leave the plants as they are because I imagine that quite a lot of your spare space will disappear during this next season. I hope these comments help Dave.