Question : Rollup summary CANNOT be used in formula field ?
Answer : No .
Rollup Summary can be used to group field values of child record .
Even thought it is formula type field , however ,
grouped field values CANNOT be cross-object formula field.
Question : How to validate the field values changed by Approval process ?
Answer : Validation is NOT a choice . Since field update fired by approval process will NOT initiate validation rule. (More explicit answer from google , Validation Rules fire before workflow rules, so the record has to meet the validation rule criteria before the workflows get a chance to run. So, the validation rule will not work when you update the workflow field.)
Tigger can be used to validate the field values changed by approval process.
