How to inherit model like this .?

Dear all,
I have 3 major questions regarding to mapping models in OpenERP (i'm using version 7)

1. there was 2 methods we can get data from table cr.execute(sql) then we can get results & add them to relevant fields.another is call model directly ex: person.salary then data will come.need to clarify what is the DB performance base better method to retrieve (retrieve,read,update,create) data.?

2. if i have a salesman model.i can add some fields from hr.employee model & need to add seperate fields also.
like extra commision rate,need vehicle.? etc...
then what is the way to do things like that.?

3. if we need to change 'account entry' something bit differ from existing procedure in openerp.then how to do that.?
'ex: need to enter records for control table and then pass it to relevant table after one month.then need to change only the relevant model.? (seems it is has some risk.bcoz going to change core of openerp)

wait 4 urs repliies friends... :-)