from my experience of slicing... I never used the 'slices by guides' feature, I always used the knife tool and the guides and cut everything up by hand. I'm not 100% what you're asking here, but it sounds like you want 'custom pieces' cut from within the 'slices by guides' feature. I don't think that's possible (tho I may be wrong). Personally after saving the main pieces out, I'd reopen the file (or work from a duplicate) and then cut the specific 'extra' pieces I need.
"Learn as much as I can at whatever cost, then give it away for free and ask for nothing in return" - (me!)