The Houdini DOS card is a snug fit in the 6100 so the processor will get hot and heat the Mac logic board. Another consideration is that the 6100 power supply is not very generous and some were unreliable when new.
On the cache SIMM question, larger SIMMs were available from third party suppliers. You should be able to offset any potential cache gain by fitting lots of RAM and a faster (low power!) hard disk.
Experiment with available 72 pin SIMMs and try to install as much as possible on the DOS card. If you attain 16MB or more, configure a RAM disk and organise your apps to use it.