As a Tank it’s common practice to use at least 5 pieces of Heavy Armor, the rest is really up to you.
By using 1 Medium and 1 Light you’ll open up your character to more max stats and a selection of new passives from the Light and Medium Armor skill lines that will improve your sustain slightly.
If you go down the route of using 7 Heavy you’ll make yourself more Tanky and benefit from more mitigation, health and resistances.
You could also go with 2 Light or 2 Medium to focus into one particular stat. Lets say you are in vCR and want more Magicka and Spell Resistance then 2 Light might work. If you want to switch your gear to Divines rather than Sturdy, you might decide to use 2 Medium Pieces to reduce the cost of your roll dodge and block cost.
The fact is, it’s up to you.