What Stella said.
But to be specific:
Games Workshop encourages kitbashing and you can often see good examples in White Dwarf.
But to be specific:
Yes.
Yes, base sizes matter. The model must be on the size base it came with.
The short answer is no, keeping in mind the wargear depicted on the model must match the equipment given to it in your list. WYSIWYG.
Games Workshop encourages kitbashing and you can often see good examples in White Dwarf.