If I’m doing an inter-service transfer (USMC to Army) and I’m a Corporal, will I keep my rank or become a Specialist?

You will enlist as a SPC. your unit after evaluating you will determine if they want to laterally promote you to the rank of CPL

If you are an E4 in the Marines then you will be one in why Army as long as you got documents to show and prove your current rank!

I went from usmc to Army as a Cpl and they made me a specialist.