In #1, the 'would' is a distancer for politeness rather than a mixed conditional, Jack (though, actually, I have so far missed any definition of 'mixed conditional').
#2, as you say, is the standard 'Conditional I'.
#3, as I have said elsewhere, I consider casual only, and the careful writer should use 'were'. The speaker does not know the size of the garage in either case.
#4 mixes your conditionals again, as you have two 'if' clauses, and the second is redundant (you can't add meta-grammar into the sentence, Jack!) 'If your garage were small, and if someone were advising you, you would be advised to do...'
In #5 there is no need for 'probably' unless the speaker is unsure of his advice; and that is not relevant to this discussion-- the unsureness is of the garage's dimensions, not the advice.
- For Teachers