In theory it is OK to connect them in parallel with two conditions: Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged.
There is series-parallel connected batteries. Series-parallel connection is when you connect a string of batteries to increase both the voltage and capacity of the battery system. For example you can connect six 6V 100Ah batteries together to give you a 24V 200Ah battery, this is achieved by configuring two strings of four batteries.
Each battery must be in a state where it can be voltage charged. This is fine for lead acid batteries unless they are very run down. Very discharged lead-acid batteries have to be charged with fixed current until they get to a minimum voltage, then they can be voltage charged. The power supply is capable of maintaining the fixed float voltage.
Connecting a battery in parallel is when you connect two or more batteries together to increase the amp-hour capacity, with a parallel battery connection the capacity will increase, however the battery voltage will remain the same. For example if you connect four 12V 100Ah batteries you would get a 12V 400Ah battery system.
With a lead acid battery bank, the internal resistances are limiting to a point that you don’t have to worry about arcing or your battery cables overheating when you connect them (not the case with lithium-ion banks…). So when we start charging, all of the battery banks are very close to the same point as far as state of charge.
In actual practice, people put lead acid batteries in parallel and cycle them that way frequently. Just look at RV's and boats and off-grid installations. A fuse for each battery would not be a bad idea. If you are charging them all anyway then what does it matter if one discharges into another?