1) It depends on how close the Perko switch is to the batteries. If the switch is very close to the batteries, as it should be, then the fusing can go between the Perko and the amplifier distribution and as close as possible to the Perko switch.
Note, if using a dual battery manual switch, everything (alternator/starter feed, helm buss & audio supply) should access power/batteries only through the Perko switch common out post and not battery direct. The exception would be the float switch/bilge pump on a moored boat.
Now an ACR/VSR scheme is different.
2) It's about protecting the boat from the supply wire based on it's gauge (14 ga.?), length and current flow. Each one of those three devices may require 1/20th of an amp to trigger on. So 1 amp should do it. I don't want to ever replace that fuse unless something more catastrophic happens so I would probably run a very safe 3 amp fuse.
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