Hey Tethys! You show up just in time!
Current limit is 30 stellar object types by the description mappings. All currently known limits however can easily be raised when we move the lexicon id tables to some other location like the relocation table you mentioned. It is plain data, no code far jumps required, just the ids need to be moved.
Some little increase would also be possible right in place when the data lookup is split or when the subroutine is moved. For now however 30 types I think already is alot.
Adding new stellar types it indeed copies current selection, but - beside that the value copy is currently broken and left empty *cough* - the hard-coded nebula image remains special to vanilla nebula type 2. I did not expect it to be worth another menu option. UE however still allows to disable the special nebula handling in the trek.exe graphics menu so the nebula is mapped too.
edit: fixed to copy current values