Suzi, I don't know how I missed this question back in October, but I'm sorry I did, and I think I can help you right now. This is a 2-part question, so I'll deal with the grain first.
The 8-grain mix is bone dry, and therefore will take moisture from the dough. The way around this problem is to take the grain you want to add to the formula and soak it in HOT water for at least 30 minutes. Hopefully the grain is already cracked. If not, run it through your food processor to break it up so the water can be effective. You hydrate the cracked grain with equal WEIGHTS of grain and hot water, and after 30 minutes press out as much remaining liquid as possible in a wire sieve. For a 2-loaf formula I recommend no more than 3-4 oz. of dry grain, and 3-4 oz. of hot water. The hydrated grain is added to the standard dough during the last 1 minute of mixing. If the dough becomes too sticky, you add a little flour 1T at a time to get it where you want it. The rest of the formula stays the same. Do not overwork the dough at this point. the cracked grain has sharp edges, and the more you knead or mix the dough, the more damage you will do to the gluten development by cutting the gluten strings.
The Sourdough Starter...
I have never worked with a sourdough powder, so you are going to have to educate me how you are using it at work. When I make sourdough bread I use about 1/4-1/2C of 100% hydrated active starter. This is my sole leavening agent, so my proof times are somewhat extended to almost twice the normal time. BUT, the flavor from the extended fermentation is worth the wait, IMHO. If you add 100% hydrated starter, just reduce teh flour and water accordingly. If you add 5 oz of starter, reduce the flour and water by 2-1/2 oz each. This is where working with a scale is most beneficial.
I send people dried starter from my personal cache if they send me a SASE. I also include complete instruction as to re-hydrating the starter and how to care for it. Just PM me if you (or anyone else here) would like some, and I'll give you my address to send the SASE to.
I hope this helps you get the multi-grain bread going. If I missed anything just let me know.