I don't think there's likely to be any interactions between the things you list. With the exception of the multivitamin, the others are things you would be taking before each meal as they need to be taken like that to help GERD anyway?
I have tried a lot of different supplements and many make me worse - multivitamins especially. Marshmallow and DGL were fine - slippery elm was so vile I don't think I could face it ever again (nausea is one of my GERD symptoms, so it was a struggle to take something that made me gag!). Zinc carnosine I need to have another trial of - it coincided with a flare up, so I'm not sure.
Everyone has a range of tolerance to these things - if I were you I would keep a symptom diary and introduce them all one at a time, waiting 10 days before you introduce a new one and reversing the last step every time you feel you may have had an adverse effect.
Let us know how it goes.
New stuff: GERD. Interstitial Cystitis
Lifelong stuff: Food allergies, eczema, asthma