Yes, the option you presented is your best choice. Destroy a cheap unnecessary building or pumping tech. Once you get that slot, upgrade terraformer, saving last slot to build another. It will become like developing a moon at this point - Very expensive, very quickly. you will be better off in the long run pushing your colonization tech, and getting another planet at some point. If you do colonize a planet, try to keep only planets that are 300 slots or more before your tech bonus. Otherwise you will quickly run into this issue. Once you have planets that are developed enough to support the loss of those with the lower slot counts, abandon them and keep colonizing until you have better planets. Watch the types of planets you colonize as well. Many can give a bonus to they type of resources that they produce if you choose appropriately.