mixing species is a crapshoot, and this issue is hotly debated among SH keepers. hybridization isn't the issue. the problem is that in a captive system, the bacterial count is VERY high, especially if you keep your tank above 75*F (which is why most peeps recommend keeping tropical SH at <74*F). SH often carry "species specific" pathogens that the carriers are immune to, but another species isn't.
i don't recommend it for novice keepers, as IMHO one really needs to get a handle on SH keeping before jumping into mixing.
NEVER mix WC and CB SH...CB's have never been exposed to all the nasties that WC have. in fact, WC's really need to be treated phophylactically for parasites as SOP.
i wouldn't mix SH from different sources, even same species.
i recommend at least 4 weeks QT prior to mixing, preferably longer.
i wouldn't mix H. ingens with ANY other species. i don't know of any hobbyist who has done it successfully. i know of one breeder in Mexico who has done it, but his facility is on the coast, and has a basically unlimited supply of fresh SW.
personally, i've tried mixing twice:
the first time, i mixed my own erectus that i raised from fry with erectus i got from OR. i ended up having to put my home-raised SH down due to a myco infection within a month (myco was confirmed by necropsy and cultures).
the second time, i got a pair of erectus and a pair of reidi from the same source, kept each separate for a a month, then added the reidi to my new 50 gal + sump SH-only system. after a month, all was well, so we added the erectus to the same tank. the male died within a month.
that being said, the female erectus is still quite happily living with the pair of reidi in that tank, so if it works, it works well, if it doesn't, it doesn't. like i said, it's a crapshoot.
i have spoken to countless people who have tried mixing, and MOST of the time, it ends badly (i've been a mod on a specialized SH forum for a number of years now). is this "scientific"? nope, but there is enuff anecdotal evidence to make me caution folks who want to try. it's better to go into mixing with your eyes open.