Wikepedia has an amazing amount of detail on Enceladus. A recent Scientific America article has a lot too. It doesn't matter how big a moon is or where it is, as much as how much variation in gravitational pull it experiences per unit mass. Enceladus has two Moons pulling on it significantly like Europa. Both have issues with the energy being sufficient however to explain an ocean. Europa is big enough for geothermal energy to be a reasonable supplement, most don't think Enceladus has a significant input from geothernmal sources. The most likely explanation is that the tidal heating on Enceladus is not steady state. It probably goes through cycles. When Europa is frozen solid the friction would be greater causing it to heat up and melt. The friction would then be reduced and Enceladus slowly freezes again. Don't remember the length of the theoretical cycle, but it's long compared to human history. We're lucky to see it in the current active state.

Europa's orbit is consistent with forming along with the rest of the Saturnian system, but the original poster may be correct that the ocean is a fairly recent although recurring phenomena.

Regards,
Mark