No, Callochromis greshakei is not a new species; in fact it’s an old name for C. pleurospilus from the area of Kigoma, Tanzania - it could have been nice with a new Callochromis spp. though
As with the Frontosa´s and Callochromis in the same tank, it’s the same as with the Xeno´s – I wouldn’t do it.
Bo, who’s a member of this board, is a very experienced Frontosa keeper who has tried various Frontosa setups; maybe he could join the discussion and direct you to some suitable tankmates that won’t get snacked on
Callochromis spp. are known to be rather aggressive, especially males towards other males, but they could also be pretty tough on other tankmates. I’ve tried keeping C. pleurospilus with other sand-dwellers like Xenotilapia bathyphila and X. spiloptera, but quickly gave up that idea, as the Callochromis were stressing the Xeno´s.
How big a sand-dweller tank are you planning to set up? If the there is enough room, some of the smaller Neolamprologus species could work out fine, but beware that N. brichardi builds colonies and have a remarkable tendency with time to spread out and be quite dominating, so it isn’t the best choice - which Xenotilapia spp. are you planning to keep? If you have some in mind, it will be easier to direct you to some suitable tankmates