Theoretically there's no harm in wiring up your gauges like that. It is electrically equivalent to making separate taps for each. However, in my experience, some electronics components act strangely when you wire their grounds in unconventional ways. I guess they were just highly sensitive to extraneous signals. But if everything works fine, you should be set.
As for the fuse, my guess is 5-10 amps is more than enough. Think how much power a 60-watt light bulb outputs; that's the equivalent to 5 amps @ 12V...
