Shading and capillary matting

Think it depends on what plants you have.

If you have got lots of seedlings close to the glass in these recent hot sunny days then they will need some, I often put up a sheet of newspaper for a temporary shade.
For more established plant perhaps a while before shading needed, but really only you can judge that .

Roof or side, it again depends on your greenhouses position and that of your plants / benches.
In mine I find the south facing side wall needs shading down to the top of the bench as well as the roof.

If using external cloth, try and lay it so there is a minimum gap of 25mm between the glass and the cloth so the heat can escape.

Last year I used Vitax greenhouse shading paint and it readily took to my polycarbonate glazing, but instead of covering it totally I tried a keen fuchsia growers tip and did 75mm vertical stripes , 75mm apart, which do give a very good dappled effect that my plants seemed to like.
The Vitax shading was also easy to remove, though it did stand up to plenty of rain, never had to reapply it.

 

I think Ricky said it all regarding shading.

Capillary matting is what is says, it carries water by capillary attraction, so you set it up on polythene with a water source, like a tray or similar full of water.
It then sucks up the water and waters your plants from the bottom.

The only things I dont like about it is, your plants are just as wet in cool cloudy conditions as they are in hot sunny ones, plus the roots do tend to grow out of the bottoms of the pots into the matting if you dont move them regularly.

 

For your main season shading, always on the outside where possible as it keep the inside temperature down, internal shading does not, as the heat has go through the glazing.

For you seedlings it really sounds like you need to do some easy to fix and remone shading as you will need to protect them on really hot sunny days but give them maximum light on overcast days.

Just a pity someone has not made some lo cost or diy auto shading that can be adjusted to suit the various growth stages .

Cannot help with the cap matting, not something I really use.
You could also try Hydroleca clay pellets, in a water soak tray of them and sit your multicells on to of it.