Certified GKH Gardening Expert
It is recommended to split questions into separate posts, so that you won't have to read a book of answers. Since they are grouped together, you will have to bear with me. This is going to be a VERY long answer. Here goes...
Pruning back sedum is a very easy task, but you will not restrict its growth in this fashion. Keeping them topped constantly will result in much fewer flower clusters, and will cause them to spread violently in all directions until it takes over the spot it is in. They will get as tall as they are set to by their genetic coding. This article will give you information on their care: https://www.gardeningknowhow.com/ornamental/cacti-succulents/sedum/sedum-plant-pruning.htm
Again, a plant will usually reach mature height regardless of cutting, but in this case, you will lose all flowering heads for oats if you cut them. You can remove the oats once they are done if you choose, but they will grow to their height either way. This article will give you care on these: https://www.gardeningknowhow.com/ornamental/foliage/northern-sea-oats/northern-sea-oats-grass.htm
Now, for the zinnia. These can be a little sensitive to root infections. This is typically caused by overwatering, or improperly draining soil. This will be cured easiest by adding dolomitic lime and wettable sulfur together. This will kill off the infection, as well as recondition the soil back to a more suitable condition. This article will help you on the care of zinnias: https://www.gardeningknowhow.com/ornamental/flowers/zinnia/growing-zinnia-flowers.htm