im wondering if i wear all light armour...then say a heavy helmet...will my moving speed and stamina in combat be affected? i cant really tell and if it is it feel subtle.
i know it makes no sense stats wise to mix n match but it might present a new way to play for me
anyone any ideas?

If you can't tell there is an issue of effect. Then you have no issue.

Shields are the only thing that don't count towards the Armor combo Perks like Matched Set. However, if you don't have those Perks, then it does not matter what armor you wear.