[Suicide Junkie]

A simple way to prevent planet colonization from making nomads powerful, it so give them

-100% reproduction and -100% environment resistance

with zero maintenance (no mod needed), and the maximum negative settings for:
-reproduction
-environment resist.
-mineral prod.
-organic prod.
-radioactive prod.

You'd be most of the way there. They would get almost nothing from planets, but would not lose ships from lack of income.

-Have a racial trait 'non-nomadic' that provides access to colony ships & colony components, and normal ships, at a cost of 10,000 racial points.
-Add another racial trait 'is a player' that gives the nomadic racial modifiers, at a cost of -10,000 racial points.
-A third trait 'is-nomadic' would provide nomadic attributes (as above), allow non-colony ship hulls, and would give special components that store resources and can be built on space stations. As well, you may want to provide access to a low-tech SM storm generator component.

So, every race needs to have the 'is-a-player' trait plus 'is-nomadic' or 'not-nomadic' in order to build ships at all.
Nomadic races would no be able to research or steal colony tech.

Now, the SE4 engine will give them Rock colonization or something by default.
That's OK, since they still can't build colony ships, or the actual colony component.

How's that for designing nomadic races? you just have to change the AI's files so they choose the racial traits 'isaplayer' and 'non-nomadic', as well as research 'non-nomadic tech I' first, so they get colony components.