I have a 12 volt compressor (Slime brand) that is small enough to put in one of my saddlebags with plenty of room left over. It came with a cigarette lighter plug, but I cut it off, and installed battery charger plugs. If I need it out on the road, I will just clamp it directly to the battery. I have a small electric compressor to use at home. The Slime compressor has it's own internal fuse.
As for the horns, are you going to replace the stock horns with the Fiamm horns, or keep all of them? If you are replacing the stock horns, how much if any more current do the Fiamm horns draw? You might be able to use the stock relay. I replaced my stock horns with aftermarket horns of the same type, from JCWhitney, and connected the stock wires to them. They work fine.
There is no reason I can think of for using a relay for a voltmeter, you might want to use a small, 10 amp or so, fuse close to the battery to protect the wire in case it should short out against something, so it won't start a fire.
However, from what I understand, all of the stock relays, in the JB, are undersized and prone to failure. If it were me, I would replace the relays with larger ones, OUTSIDE the JB.
I used to do a lot of residential A/C work on the side, the old units had separate heavy duty relays for everything. They were easy and inexpensive to repair/replace, and didn't fail that often. Newer units have everything, including the relays for the indoor and outdoor fans, on a made in Mexico circuit board. They are seriously undersized, and fail on a regular basis. Due to the overall design, it would be extremely difficult to install proper relays. Fortunately, it is an easy fix, just replace the entire board, which is shared by many different units, for about $200, give the old one to the customer, explain what happened, and tell them that unfortunately, due to the lack of quality and size of the relays on the board, there really is no way to repair it properly, and they can expect another failure at some point in the future. I almost always tried to talk someone into repairing an old unit, but most of the time they wanted a new one, even after I explained to them that most new units were cheaply made junk. For some reason, most people seem to think that new means better, when in fact, it is almost always the exact opposite.
I have an old unit made in the early '80s, and it has no electronics on it. I have replaced both fans and a defrost timer, but never had a relay fail. I have installed both liquid line and suction line filters, so hopefully if the compressor had a hard failure, it won't contaminate the entire system, and can simply be replaced. I want to keep this old unit going as long as possible. Sorry for getting off topic. Jerry.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike