Can wet-cell and gel-cell batteries be mixed?
By "combine," do you mean "charge and discharge in parallel"?

I'm assuming the basics are the same -- the voltages and charge-top and charge-bottom cut-offs are the same, and the Amp-hour ratings are matched.

In general, this will "work" for short-term capacity boosts, but trying to use the same charge/management circuitry for multiple chemistries in parallel is likely to make the cells skew over time. Lead/Acid is less sensitive than Lithium-based systems, though -- you never see "Balancing wires" for the series cells in a car battery.

In general, for the longest lifetimes and best control, you want one charge management circuit per battery chemistry, and ideally per battery (there may be benefits even within the same chemistry.)