Do I stick a slightly smaller cassette on a 29er. A >32t feels a bit over geared as it is on a normal bike.

Bigger cassette for similar gearing to 26″.
More wheel to turn.

Thought the general idea was to go up in size i.e. 12-36 tooth not down as the extra force needed to spin the wheels up or something like that.

I ride mine with a 32 having said that but am just about to try 36 tooth to see if it really makes all that much difference on the climbs.

I have a 36 on my bike
may go 2×9 in the future as only used the granny ring once

You gotta go 36t for sure

A 29er wheel with tyre on is almost exactly 10% bigger in circumference than a 26″ wheel with tyre.

The upshot of this is to maintain similar gearing, you need to go for 10% bigger sprockets on the back (12-36 cassette generally), or go 10% smaller on the front. When I built my 29er with Alfine, I looked at what the 26″ guys were running first (which seemed to be a 34T sprocket with 20T cog mainly), then fitted a cog 10% bigger on the back (22 with the 34T ring) to get the ratios I wanted.

If the difference is two teeth, ie: 32/16 on a 26er becomes 32/18 on a 29er, then the easiest way is to drop two teeth at the front…?
A 36t cassette might give you the same hi gear, but the the 11t at the other end is going to lose you some speed ?
If this is total bllx its because I’m old.

+2 teeeth on the rear = roughly the same gear.