The best thing, I think, would be to play a whole lot of arcade or arcade-style games, and see how they work, see what makes them fun (or not), and go from there. With MAME (Multiple Arcade Machine Emulator), the possibilities are endless, and an entire generation of gaming can rest in your laptop with no real effort.
Remember that the thing with shooters is that the longest shooters out there are from this generation and previous, and even then they are only forty minutes to an hour long. The more refined your game is (see CAVE's latest arcade ports, such as the now very-cheap Shin Akai Katana and their upcoming DoDonPachi SaiDaiOuJou on the 360) the shorter it will be - anywhere between 20 and 30 minutes.
Psikyo's Dragon Blaze is only 15 minutes long, but hot damn is it some of the most glorious fifteen minutes of gaming out there!
When you are making an arcade style game, it needs to have the right type of pace. Notice in all the examples you've listed, things are always clipping along nicely. There are quiet moments here and there to catch your breath, but there's no "filler" or if there is it lasts at best 2-3 seconds and that's once a level. Modern game design seems to dictate that you need lots of hours of content in a single play to have a meaningful game (and often this content is boring padding), but the old shooters are about short play times, with lots of replay value - competing for score is addicting.
Split games are a mixed bag. In a shooter sense, the NES game Guardian Legend is a pretty cool hybrid of zelda-adventure and shmup. You have Wonder Boy III on the arcades and Genesis that is a cute, fun platformer with annoying and frustrating levels. If you're going to do a hybrid, make sure they both work damn well, and that they compliment one another.
As for weapons, in an arcade-style game, the player-character is everything. The entire game must be designed around how the player-character moves, reacts, and fights. So the player and weapons need to be designed first, often. An overabundance of weapons can complicate things really fast. Originally the developers of R-Type were going to have your Force Pod attach to the top and bottom of your ship as well, but that made things far too difficult. A well-crafted simple system is better than a mediocre system bustling with features.
As for level length, my thoughts about game length in general should apply. Basically, you don't want to bore the player with long stretches of un-fun or un-challenging segments. A minute shooting asteroids slowly drifting towards is a sure-fire way to lose the interest of a player.
Again, this is where playing some of the classics and making some observations is handy. In the Taito masterpiece RayForce (Arcade, Saturn, PS2 in Taito Legends 2) each level lasts about four to five minutes, but a lot is going on, and the later levels are longer and more challenging. In a game like Strikers 1945 by Psikyo, a level can just be a few waves of enemies before a boss encounter. Don't feel too tied down by level lengths - a good game that takes 10 minutes to beat is worth a whole lot more than a boring, meandering 10 hour game.
- Anything mentioned in the above post
- Darius Gaiden (TAITO)
- Twin Cobra (TOAPLAN)
- Batsugun (TOAPLAN)
- Radiant Silvergun (Treasure)
- Salamander/Life Force (Konami)