Yeah, my players didn't realise that at the start and got quite a surprise when I explained it to them. As a result, only 1 character out of 6 has taken levels in a class other than their initial one as they all want a taste of epic classes. The one that has diversified (the cleric) took all 5 levels of Thaumaturgist before returning to cleric levels. Technically, he won't qualify for epic cleric until he's taken 20 levels of cleric but in a moment of weakness I decided to cut him some slack and agreed that his Thaumaturgist levels could count as cleric levels, seeing as he got to the maximum level of Thaum and it's a prestige class that stacks well with cleric.
As far as supplement books go, I made a ruling at the start that we were playing core books only. Anything setting-specific (Eberron, Forgotten Realms, etc.) was banned. Anything from other supplements had to be approved by me in advance. Apart from a couple of spells from Complete Arcane and a few more from Races of the Dragon and Dragon Magic (the party sorcerer is a Spellscale from Kandang so it seemed reasonable to allow him some draconic magic, given the history of that nation) I don't think I've had any requests for any other spells, feats, skills or classes. It's worked well as, in my opinion, the players have enough else that's new to them with the Starborn powers that they don't need any more complications.
TC