I am not sure really where a post like this would belong, but I had a question that I could not find the answer to. How does marking time apply to all time signatures? For instance, in meters where the quarter note has the beat (e.g. 2/4, 3/4, 4/4), the feet hit the ground on every quarter note. But how does that apply to meters where different notes have the beat? (e.g. 3/2, 5/8, 25/16) Some meters can be thought of as other meters in terms of the feet (12/8 -> 4/4), but how would one think of a meter such as 25/16? How do the feet apply?

It's all up to interpretation. If the music is written in a odd time signature such as 25/16,17/16,etc. then in my opinion it all really depends on the music. Mark time to what feels right for the music. If your talking about show music or a cadence for high school or college then ask your instructor what he wants. Usually you'll be able to find a comfortable way to mark time. Try every 4 sixteenth notes, every 3 sixteenth notes or every 8 sixteenth and carefully look at the music and determine what would fit best. Just keep going from there and you should be able to find a good consistent tempo to mark time to even if the music feels weird marking time.

that makes sense

It is definitely open to interpretation; however, typically your answer can be found by seeing what pulse your conductor / drum major is using. If the conductor is pulsing the 16th note, then your feet should be in time with the 16th note.
There are certain exceptions... if you are interested in creating an effect such as having the a section of the ensemble march in a different time from the rest of the ensemble, or if you are marching an uneven time such as 7/8 conducted in 3 (ex 2+2+3). Uneven times can create a real issue as it makes the ensemble appear as though they have an odd hitch in their step....In this case I typically examine options for adjusting the meter to ensure that the pulse remains even.