Deafulst Stock or Inventory

Please guide me i have some problems...

1. when i am creating 3 POS and the 3 user for each different default stocks, the problem is when a user is login he also can select another stock , i want to remove other stocks for which he is not allowed to sale and atleast if it is not possible then his default stock should be that one which is selected during his account or POS creation time because when he doing mistake the goods are sold form anohter stock or either there should be a solution to ristric sale person form sellding from another stock.

2. i have three different shops who are taking goods from centeral stock which i am managing, i want to keep tracke of their sale, and stock condition for how much each one took. if it is possible with dimension then please tell the very basic start although some information is given on the forum but it the information as one already should expert not for novice.

3. there is also validation of data is not applied in FA if you enter characters instead of digits FA is accpting and some time create very big problmes like can make no of goods in stock 0.

Regards,