Your printer is an ideal candidate for what is known as P2, in which you effectively run K6 rather than K7, leaving out shade 7. This allows you to put glossy shade 1 in the Yellow slot, and the P2 curves for glossy papers were designed for this configuration. My experience is that you don’t loose much by leaving out shade 7 - I’ve never managed to spot the difference - and you gain a lot in flexibility.
What would be ideal would be a universal shade 1 that you could use on both gloss and matte. The risk of using MK on gloss is that it may rub off, and the risk of using PK on matte is that you lose dmax. There [I]was[/I] a universal shade 1 a few years back, not designed that way but it was discovered to work that way, but in the way of things, it was reformulated and lost that property. There is a whiz-bang new black ink coming out that may reinstitute this capability. Walker will clarify this no doubt.
In a 3880, GO would normally go in the LLK position. I think you have the space for it. Yes, glossy & baryta papers have a higher dmax than matte and hence a different look. What look do you want?
p.s. I see that Walker has also suggested P2, although I wonder it he actually meant to put MK in the K channel as per usual and GO in the LLK channel.