Finally I created several css classes that I add either to element wrapper or to the input element itself, and I position each element accordingly.
If your form is not too long, and you don't mind creating other webform elements, you might want to wrap the fields that you want to put on the same line in a fieldset, you give row-fluid class to the fieldset, and all is good. This obviously also depends on the functioning of the form, some modules don't work as expected with nested fieldsets, ex. webform add more module (at leat las I checked it didn't, was about a month ago)
In my case I didn't want to do this, as the form is long and complicated and it could've broken some functioning.