1. Yes. Humidity trays will help raise the rate of humidity around your orchid plant. The amount or level of increase will vary and be determined by the number of plants you have grouped together, how close they are to each other, how large of a humidity tray you have, and how deep. Air movement is also a factor that you should take into consideration when thinking about humidity since they often go hand in hand.
2. Sure. You can care for them the same way as your cattleya. Some get large and some --not so much. (Depends on what they have been crossed with and the sizes of those plants.) Also on how well the plant is cared for regarding food and fertilizer, etc... The bag babies sometimes are 'close' or 'near' blooming size, but some can take three or more years to reach the maturity for blooms.
3. Cattllyas like to dry out between watering. Standard catts like bright light, but not direct light. Mini-catts will do well in a bit less intense light. If you summer them outside in our area, you will have to watch out for bugs and keep an eye on them when it is raining often. Be sure to keep them in some protective shade.