... I could open it with 1.29 beta 2?
Yes. Sorry if that wasn't clear.
Do I understand that you did not play the 30 civ scenario?
Correct. As I said, I'm no editor expert, so if I create one and it doesn't work I don't have the experience to work out quickly whether it's because of the civ count or because of something else I did or failed to do. As I don't have time for that learning curve right now, I tried to delegate that to you
The next paragraph refers to some of teh data structures in a scenario file.
Each RACE element defines the characteristics of a tribe, eg The Romans.
Each LEAD element defines in-game information for a player, like name, score, wealth, contacts, etc for a Leader. Each Leader has a RACE, and more than one Leader can be Roman. Confused? If not, why not?
There are always 32 available LEAD (ie. Civ) slots in the game, numbered 0 to 31, each pointing to a RACE record in the BIC, or to -1 if unused. The barbs take up one slot leaving 31 for competing civs. The standard BIC only knows about 16 RACEs as you say, so you can only insert duplicates of existing LEADs to make up LEAD numbers greater than 16. The GOTM mod adds another bunch of RACEs, as does the DyP mod, but they are both version 1.29 BICs, and the editor will crash if you try to use them as the basis for a new scenario. Your mission, should you choose to accept it, would be to add some of these RACEs to a 1.21 BIC, then create a >16 RACE scenario in the 1.21 editor. I suspect it's possible, but it may not be straightforward.