Perko is a brand name. There are many different brands of battery switches.
You really don't want a "loose" battery. If it is a flooded battery and it tips over you will have a real mess. It is also very important to not let anything short out the battery.
I don't understand what you want, you seem to contradict yourself in the second and third paragraph. Do you want the second battery to be a "reserve" or do you want the stereo/amps on a battery of their own?
A reserve battery doesn't do you any good unless it is charged up when you need it. A lead acid battery will self discharge, so unless you have a means of keeping it charged it will go dead in a few months of non use. It would be feasible to put a charger on the reserve battery when you were at home, perhaps once a month or so, but an automatic system would be better.
Likewise, a separate battery for the stereo will need to be charged up whenever the engine is running. A typical method is to use the the battery switch to connect both battery systems together when the engine is running and then isolate them when the engine is off. The manual battery switch can do this but if you forget then you can end up with a problem.
A "Battery Combiner" is similar to a battery switch and it automates the battery switching process. It senses when the alternator is charging the primarey battery and will connect in the secondary battery when the charge voltage is high enough. Simple and foolproof.
No matter what type of switch/combiner you have the wiring is similar. The negative side of the batteries should be connected together at all times. The primary battery should already be tied directly to the engine block, and it is recommended that the secondary battery also connect directly to the engine block (instead of battery to battery).