When you put "choose(1,2,3,4,5,6,7)" inside quotation marks as you have, it is no longer an expression, but a string.
I don't think you can have an expression inside a variable. Are you trying to have the random number picked when you go into the next layout? Or have the next layout pick the random number?
You can:
1. Use an action to set your GLOBAL_MONSG variable to choose(1,2,3,4,5,6,7), in which case a random number whole number between and including 1 and 7 will be stored in the variable. Then you can set RandomMonster to GLOBAL_MONSG and get the same number.
2. Use a function that returns "choose(1,2,3,4,5,6,7)", and have your action set your RandomMonster variable to your function output.