IS it true that you cannot network an NEC sl2100 with an SV9100? I find that hard to believe.
Correct. This is one of the PRIMARY differences between the systems besides sheer size. There are many types of networking bit Netlink is probably what you are looking for. That requires all processors to be the same.
Also, the SL2100 does not support IP-CCIS, sections 50 (CCIS) and 51 (Netlink) do not exist in the 2100 programming. No way to connect it with the 9100
SL2100 networks to other 2100's via SLNET (up to 50 sites)
SL2100 does not offer CCIS
There are always workarounds if you are willing to pay for the licences!
IP trunks between systems using LCR springs to mind.