Hi 

I am only an Odoo user and sorry for the generalities in the response. 
Unfortunately, this is how the rule you described works: 

1.If there is enough, take it from stock 

2.if there is not enough - call a rule that will provide the ENTIRE quantity that is needed for a given call 

If you want to perform an action with all points: 

1.if there is NOTHING - call another rule
2.if there is enough - take it
3. if there is a part - take this part and call another rule for the rest 

then unfortunately you have to write your own path and your own rules to replace MTO. 

Creating rules itself is not difficult (you need to start by reading the user documentation for the warehouse on Odoo websites). Unfortunately, I can not write a rule above "improving" the MTO - which you need and this task is probably not easy. Additionally - how you do it depends on your warehouse settings (e.g. whether production is defined with raw material release and finished product return documents, whether your warehouse is divided into zones ...). 

The best way is to contact one of the companies implementing Odoo in your country / region - they should be able to configure the warehouse and paths.