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Collimation will have a huge impact on the quality of the image. However, once collimated poor seeing will also severly impact on what you see. Mars has a 97% phase at the moment and is slightly egg shaped.
High magnification should only be used in very good seeing otherwise no further information (detail) is visible even though the image is larger.
Now a 10mm on your 200mm telescope and assuming 1200mm focal length would give 120x which is quite within the range of your telescope. However, Mars will be quite small at this power. 160-200x would be a practical limit for nights of good seeing and 200-240x would be a limit on nights of very good seeing and 240-300x on nights of exceptional seeing (rare). To get 300x you would need an eyepiece of 4mm and this would give you an exit pupil of 0.75mm. When the exit pupil is 0.5mm the image starts to become a little too dim but OK. Lower than 0.5 and you need more aperture.
Exit pupil = focal length of eyepiece / focal ratio of telescope.
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