A few quick questions about building an amp...

1) What gauge wire is used for most stuff inside an amp? I was thinking 20 gauge wire is the way to go, but I'm not sure.

2) Does it matter what kind of solder you use? I have a roll of .050" gauge 60/40 rosin-core solder and am just making sure this will do fine.
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More than gauge, worry about insulation strength, 600v is typically used, 300v is common, but if using more than 300v you'll need 600v. 16-22 gauge is fine. 12 is used for ground bus wires. 60/40 is the standard with electronics.
i use 20~22 gauge stranded 600v PVC wires. If the amp has a few EL34s, i'd use 18 gauge solid core wires for the heaters.
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I use 18ga wire for the heaters - green and black
20 gauge for the power supply - red
22 gauge for the rest of the amp, in various colors depending on their purpose, as an example all of my cathode uses black wire and all of my plate wire, excluding the power tubes, are red. I also use blue and yellow, alternating them by phase of the signal.

All Teflon coated wire.