3.Though not more than one form will be active  all the time? does it consumes  more device  memory if we use that many forms?

Always try to keep open parent form and keep closing the child forms  and try to use single ton pattern. This way does not consume much memory. Dont come to the conculsion whether the application is slow by testing only on emulator. Please test it on device as device is faster than emulator.

1.It’s not quite normal to maintain an instance for every form  in your application  at the same time. The users never use them at the same time; instead they usually use a small portion of them within a certain period of time.

The first option you can use to optimize your application is using “lazy initialization”, which means you do not create an instance of a form until it is being accessed.

Another option is applying some kind of caching policy. For example, you can choose to maintain a maximum of 10 form instances which are most lately (or frequently) accessed; or you can dispose a form after it hasn’t been used for a certain period of time.

2.Using switchable user controls should save some memory, though I don’t think the difference is significant, so maybe you can just choose the one you are more familiar with. Whatever way you choose, you need to make a compromise; and the Remote Performance Tool will help you in this regard.

Besides, there is a Compact Composite UI Application Block included in the Mobile Client Software Factory which aims to help you build a better UI. You don’t have to use it but the guidelines might be helpful to you:

http://msdn.microsoft.com/en-us/library/aa480471.aspx