1. Don is as strong as Bill.
2. Don is much stronger than Bill.
3. Don is a little stronger than Bill.
4. Don is somewhat stronger than Bill.
5. Don is less strong than Bill.
6. Don is much the best strongest of the three.
7. Don is the very best strongest of all.
(Are all the expressions grammatical?)
8. My father is only older than my mother. (Is this grammatical? What meanings does it have?)
9. It is getting cold and cold outside.
(is it correct?)
1 - 5 are correct; 6 and 7 are not.
8 will be fine if you remove "only."
9 would be fine if you write "colder and colder" in there.