[Top Down]

Should you measure the internal diameter, or to the edge of the rim? 

Some inserts go IN the wheel, others seem to sit ON and cover the rim.  

Particularly thinking Slot.it and NSR. 

Will inserts from one brand fit the other or is it best to stick with the same brand of wheel and insert?

[Kevan]

I'd say it's your choice.

I like inserts to go inside the wheel.

[Savage GT]

Generally its inside (slot.it, NSR), although the newer Slot its will fit both 15.8 and 16.5, inside on the 16.5 outside on the 15.8.
Sideways are outside fittings.
NSR and Slot.it are slighty different sizes but you can usually make it work between them with a bit of sanding or filling.
Like for like is easy, NSR and Slot,.it will tell you which size from their range the inserts fit.
Sloting plus are again a little different, so its often a bit of trial and error to find something suitable for those.

Generally the inserts with just a centre mount are more versatile as they can accommodate inside or overlap fittings.

[KensRedZed]

I'm with Kev on this one. I prefer inserts to go inside the wheel.

   

[Kevan]

Thanks for naming your image Ken, that's a lovely '66 olds 442

[KensRedZed]

I humbly thank you Kevan. You are very kind.

It is indeed an "Olds mo boat".