I don't think the Founders really had a default humanoid shape - or, at least, they didn't until they came across Odo.
If Odo's appearance really was typical of the Founders, it seems like the Vorta (Erris, Emmis, maybe?) from the Season 2 finale should have recognized him as one of her gods. Instead, she barely acknowledges him when he steps up to arrest her before she escapes.
Since the Founders make their first appearance in the next episode looking like Odo, I always assumed that they were able to somehow determine that Odo was one of them from the Vorta's report and thereafter mimicked his appearance to make it clear to him and everyone else that they were the same species.