a) You don't need a charging management system, the alternator will cope with two batteries in parallel. What you do need is a discharging management system; see point d)

b) gel types aren't necessarily smaller capacity (Amp hrs). As long as it's 12V it will be OK. You will need to use a gel or AGM inside the cabin though- not a wet cell.

c) you will need to feed the cable through one of the rubber grommets in the firewall. Tools required will include bandaids for your knuckles. Alternatively, you can run the cable under the vehicle and enter the cabin via the space behind the rear wheel arches. I think there is a grommet there too.

d) a Voltage Sensing Relay (VSR) will disconnect the two batteries when the engine stops and the voltage falls below about 12.4V. It will reconnect them when the engine runs and the voltage reaches 13 point something. When the second battery is connected to the main battery, it will also be charged by the alternator. When disconnected, the accessories connected to the second battery won't touch the main battery.
I reckon it's too much trouble to run all accessories e.g. car radio etc., off the second battery; that would take a lot of rewiring. Usually it's just the fridge and maybe some camp lighting that you would run off the second battery.
A VSR will cost around $100. Make sure you use cable that's up to the task, otherwise voltage drop will be a problem.

 

A VSR is not the best choice if connecting two different types of battery.

You would be better off using a DC/DC charger - essentially using the charge in one battery to charge the second battery.

Use thick cables and take care to ensure that all connections are good and solid.