The "purists" would say "dont mix different chemistries". But in your case, you would be fine with a standard automotive battery for a starter on the genset.
The reason is...
The EchoCharge. It only "echos" when the main bank is topped up, then it "trickle charges" the second battery (genset starter). Since your genset starter battery is only used for "starting" it will nearly always be at "full charge" under normal use. The charging
voltage for both gels and flooded at near full capacity is not very different (if at all), so concern for "charge profile" is not warranted (the primary reason why you dont mix chemistries: different charge profiles).
Now, if you were using your genset starter battery for other regular uses, to cause deeper discharges, it would be a different story.
My opinion of course. I am sure there is someone here that will disagree.