Adding grains to Coopers Stout Kit?

I am going to brew up an oatmeal Irish stout, using the Coopers Irish Stout kit. It comes prehopped, and apparently pre-grained as well. I have an additional can of light LME (3.3 lbs), and a pound each of patent, roasted barley, and marris otter. I am going to steep the oats, but if I throw in say 1/4 or 1/2 pound of each of the grains, will I be overdoing it? Also, how does the prehopping balance out? I have a mix of Glacier and Galena pellets that I can throw in to up the hops, but again, am I overdoing it? Lastly, when do I add the gypsum?
Thanks

Are you substituting the LME for sugar? If so, I'd recommend just steeping the oats and not making any other changes. Stouts generally do not have much of a hop aroma.

If the gypsum was included in the kit, it can be added at any time. Otherwise, you probably do not need it because the kit will have had its water modified during the mashing process.

I am substituting the LME for sugar, yes. I will try just steeping the oats, and add a little gypsum because, what the heck, I have it. I will save the grains for a future stout or 2.