phono channel output

Are phono channels of integrated amps typically lower in volume?

Creek Classic 5350SE Integrated Amplifier, MM phono channel
Rega P2
Dynavectot 10x5 cartridge
Lower than what? CDP, maybe. Your Dynavector has 2.5 mv output, so it should be high enough to drive the MM phono stage, but not necessarily as loud at some gain setting as other inputs. A lot depends on the other source voltage output and its not standard.
Much lower than the CDP or the ipod.

If I play a record, then switch to a CD, I have to turn down the volume a good amount.
yeah , that is very normal
IMO, yes, it might be normal, but that doesn't make it likeable-desireable, as it's apparent in Rb's case.

I'm thinking that these modern day integrated MM stages are still adhering to the old MM standard, in which most MM Cartridges of yesteryear typically put out more like 4-5Mv, and not 2.5Mv

I personally wouldn't like this myself, and in fact have run into this problem, but a slightly differnt twist in which it was an MC Cartridge, and an incompatible Phono Pre.

Naturally no, one wouldn't want a Phono-Stage to be so hot, that you might clip, but still, a gross mismatch of gain to cause one to crank a volume control quite high, and then constantly have to remember 'Oh, I have to turn down my volume, otherwise I'm going to get a nasty, shocking surprise", IMO is a hassle.

There is probably some way that you might be able to have the gain increased in the Phono Stage, and the other options are obvious... switch to a different, higher output Cartridge, or Phono Stage-pre-amp that has better gain adjustability.