You presume correctly. You will have to leave one in the forge until Goal 9. In the meantime, you can place your first one in your realm, and make it grow into a Sunbuddy.
It is only when you reach Goal 9 - Raise a Sunbloom, you put your grown up Sunbuddy back into storage, then craft your first Sunbloom. Only then you can take your second Sunbaby out of the forge, and you can decide whether:
- to keep it as a Sunbaby
- to grow it as a Sunbuddy
- to craft a second Sunbloom
Good luck!
Note: in case you accidentally visit the forge, you will have the option to sell your second Sunbaby (for 0 coin!)
if this happens to you, simply restart your game (the Sunbaby will be put back in the forge)