A tricky and untested way, but should be effective
- Set Colony Pod equipment pre-req tech to "unresearchable" (whatever name is used for cant be built... same as "slow")
- Set Colony Pod equipment cost to 99 to discourage reverse engineering.
- Set colony pod unit cost to '3'
- In pre designed units section, create any other colony pod units you may want, like a drop colony pod and a fusion rover colony pod and put them at appropriate techs and at appropriate costs.
With any luck this will have the effect you want, if you try it please report back here and say if it worked