NO. Do NOT mix and match battery chemistries and charger technologies. NiMHs and PbAs require *totally* different charging regimes. You will destroy your batteries for sure, possibly your UPS board or your Mac.
The *only* exception to this is that NiMHs and NiCads are similar enough that they can *usually* work in the same charger. LiPos and LiFePos have a similar relationship to each other (*only* to each other, *not* to NiMHs and NiCads) but given their tendency to splode, I would only recommend charging each type in a charger that *is guaranteed* to work with that type (ie, you can get chargers that do both lipo and lifepo).
Uh....
NiMHs are 1.2V each. Four of them = 4.8V. 16 of = 19.2V. Neither of these is 12V. For 12V, you need ten, in series.
I would suggest either of the following courses of action:
• Use as large a 12V SLA (sealed lead-acid) as you can fit [*] in the Mac, a small MiniITX DC-DC board (and a thingum for -5V if required), and an external, readymade SLA charger (one rated for SLAs, not car batteries)
• Step away from the hack and research moar.
[*] Remember, with the mITX DC board, you can remove the SE/30s original power supply for even more room.