Make a space platform in space age, place a nixie tube on it, make a blueprint of the space platform.
Make another platform using the saved blueprint and while everything else gets built, but the rocket silo on the surface will not pick up nixie tubes.
I suspect this may be down to an issue with the calculated weight for rocket capacity even though the new rocket capacity in the selector combinator does yield a value (1.3K+) and I do have that many in a logistics box for testing.

It appears that cause is that the low weight of these items causes the rocket capacity to be so high that a rocket silo cannot fit enough of them in to fill a load.
I would suggest look at one of the combinators and set the weight to be double one of them (for eg).

the rocket capacity is one stack

The pod capacity is one stack, however the silo will not deliver until it has a full amount in the silo with the result that platform construction remains stalled.
The silo itself only can store a limited number of stacks and that limit is too few to hold the 1.3K+ required to fill the weight quota.
Personally I think the game behaviour is broken (by design) in this respect, but it is what it is.

I expect setting a weight will solve the issue, for eg from base\prototypes\item.lua in selector-combinator has:
stack_size= 50,
weight = 20*kg

Some items have ingredient_to_weight_coefficient instead that I assume control the default weight calculation.

What I do not get is I thought item weights were also auto-determined from ingredient weights unless overridden so you didnt actually have to specify an explicit weight, however in this case that seems to not work correctly for whatever reason and yields a rocket capacity that is too high to fit in a silo.

it works fine. you've got some other mods screwing stuff up.