mix different batteries in parallel

You need to look at the specifications for the two battery types.

For example, depending on the exact Victron GEL battery you have, they are charged at 14.0 to 14.2 volts or 14.1 to 14.4 volts.

GEL batteries cannot have distilled water added if they are over charged and the vent... The venting loses "water" (hydrogen and oxygen gassing) and will shorten their life if "excessive" charge voltage.

Are your other batteries "Everstart" (vs "Eveready"?) which (as far as I can tell) are flooded cell Lead+Calcium batteries... The Calcium generally reduces water usage (less gassing).

The dual cycle batteries have cell caps where (distilled) water can be added as needed. In normal operation as SLI (starting/lights/ignition) batteries--They may never need water added. If operated as deep cycle batteries with "fast" or "higher voltage" charging, they can gas more and need distilled water (as needed, prevent plate exposure to air--Will "kill" FLA batteries very quickly).

Typical alternator charging system (cars and such) seem to charge around 14.0 to 14.4 volts (at 77F/25C). For "deep cycle" flooded cell lead acid batteries, they are typically charged at ~14.75 volts (for 12 volt system) and can be charged as high as 15.0 to even 16.0 volts during equalization charging ("higher voltage" controlled over charging to bring up "weak cells" to 100% State of charge along with the "good cells").

AGM and GEL batteries are usually never EQ charged (although there is a suggested EQ for AGM (and GEL?) batteries being EQ'ed for ~8 hours or so at their 14.2 or 14.4 volts once every 6 months to bring up any "weak cells").

I could not find any charging voltage specifications for the EverStart FLA Dual Cycle batteries...

In theory, if you keep your charging voltage for your bank at the GEL voltage setpoints... They will probably "play OK" together (at the cost of "slower charging" and "charging to less than 100% SoC" for the FLA batteries). If you see capacity issues with the FLA string, you may wish to "EQ" the FLA string (elevated EQ charging voltage) as needed.

I would put "like batteries" in the same series strings (your Victron GEL batteries in "string A" and FLA batteries in "string B", etc.if this was a 24 or 48 VDC bank, or if these were 6 volts batteries in series for 12/24/48 vdc banks).

In general, getting 5-7 years for your battery bank is usually pretty good service life (for smaller AGM/GEL/Golf Cart type batteries being cycled daily).

If you have a choice next time... I would suggest larger AH batteries (i.e., instead of 12 volt @ 100 AH batteries in parallel vs 6 volt @ 200 AH in series parallel--i.e., 4x 12 volt @ 100 AH = 12 volt @ 400 AH battery bank vs 2x 6 volt @ 200 AH in series * 2x parallel strings for 12 volt @ 400 AH). Besides "fewer" parallel strings of batteries (I prefer that), and fewer cells to check specific gravity/water levels for FLA batteries,), you can measure the voltage of each battery (i.e., every 6 volt battery in a 12 volt bank) with a voltmeter to ensure that the batteries are working well and are balanced state of charge.

When you have 12 volt batteries all in parallel--You cannot use a voltmeter to quickly check the health of each battery--All batteries paralleled have the "same measured voltage". You have to disconnect the batteries to measure each battery's voltage (and rough health). Kind of a pain. 

Anyway--Those are my guesses...

-Bill