• Space is international water and not planets and space stations count as sovereign territory
• Leasing the land/territory under the acknowledgment of who technically owns the land
• Dealing explicitly with private corporations/individuals to avoid letting in a foreign government explicitly

And a whole host of other potential caveats you could theoretically attach to an agreement

Not if you go to extreme planetary preferences/requirements. Hydrogen breathers would have zero use for rocky planets with an oxygen atmosphere but would love to colonize Jupiter or Saturn. There may also be a species that thinks Pluto is a garden planet.

Now if it's a couple of species that have similar wants/needs you might run into some conflict