Hi roh mih,
I find T's question to be the one that comes to my mind first. Why do you want to create a new form? What is it that you want to accomplish? What are your training goals, if any?
If inventing your own short form would increase your motivation to practice and research, I think that would be a good reason. If you want to do it for enjoyment, that would also seem to be a fine reason to do it. If, however, your purpose is to increase the effectiveness of your practice or make faster progress, I think it is probably not a good idea.
I think that the better forms have many layers that might not be apparent at first examination. They can have built-in warm ups, deliberate changes in rhythm, opportunities to challenge your practice, or moments to rest up. I personally find even moral lessons in parts of the form where I perceive that escalation (or the lack thereof) is linked to certain circumstances.
Designing good forms is very difficult. Many of them are casually described as the work of individuals, but most have needed several generations or the work of many individuals to work out most of the kinks. Also, all forms do not serve the same purpose. The better ones play a specific role within the broader context of a curriculum or a training method.
<font face="Verdana, Arial" size="2">Until I read your reply, Cesar-- quoting Yang Cheng Fu saying "repetition of earlier moves has a very good regulating function"-- I didn't quite understand before why there has to be repititions of such forms as Repulse the Monkey and Cloud Hands, etc.</font>
I have speculated about other reasons for the repetitions, as well. One reason might be to balance out various aspects of the form. For instance, if the form is to begin and end in the same place, you must balance out the directions of movement.
Another reason might be to balance out muscular usage. If, for instance, you deleted one of the Cloud Hand sequences from the traditional form, you would be substantially effecting the percentage of Horse Stances in the form. This has implications for muscle strength, flexibility, and development, especially if you are talking about long-term or intense practice.
Another reason might have to do with allowing practice within practice. By repeating certain moves, you can try to work on deeper levels without having to worry so much about the basic foundation. This is probably why three reps of the long form can be so effective. You get to set the bar higher and can go deeper each time you repeat a posture.
Another reason for repetition is to show moves in different contexts and to open up possibilities. If the entry into a posture or the exit from it are different, is it really the same posture? Even if you consider it the same, does it not allow you to explore more properties of the posture? For example, Single Whip is a very open posture that can be matched by different ways to close it: e.g., Lifting Hands, Cloud Hands, High Pat on Horse, and Snake Creeps Down.
<font face="Verdana, Arial" size="2">In practice, I do a short form first (either the 24 or the 40), and then the long form.</font>
I have heard of people who normally do the traditional form followed by the 49. I had presumed that this order was intentional, to get around the lack of "warm up" in the 49. Why do you do a short form before, rather than after the long form? What is it exactly that you prefer about the short form or about combining it with the long form?