Fairly certain you need at least Kickstart 2.05 for the A600 in order to support internal IDE header and PCMCIA slot. I assume you've already tried all the normal WHDLoad/Degrader/etc. options for your old games? AFAIK a few very old games require 1.2, possibly even earlier...
Also - not the same memory setup. An A600 is going to have 1MB of chip ram (again, unless you use some Degrader-like utility to cut that in half), so some games might not work right when they see that.
