1st, custom moves, I use 1231 or 1331, is this good or bad? Normal klaw is horrible imo compared to the other two.
I want to refrain from flat out stating what's good an bad, especially when the game is new, so now's the time to experiment! I will say that I personally feel that Dash Klaw is the best Klaw. It offers mobility and a great mixup tool.
I actually dislike Hit Klaw the most, because it's weak and only sets up for combos are extremely low percents. Dash Klaw leads into more damage in nearly every situation, and offers the same great positional advantage. Also, Dash Klaw is a command grab while Hit Klaw is not. The advantages are very clear.
I will say that I feel that regular Klaw actually has more of a place in Bowser's gameplan then Hit Klaw, on the sole grounds that Dash Klaw is better then Hit Klaw in nearly every situation. Regular Klaw can still be used for the old fashioned mixup approaches and shield baits while falling on opponents. This is impossible to do with Dash Klaw or Hit Klaw. While Klaw itself is heavily nerfed from it's Brawl incarnation, I still feel it has a place if that's your thing. Regular Klaw also sets up for somewhat better punishes out of a run compared to Bowser's other alternatives.
I'll talk about Fortress in a bit.
2nd, regarding normal moves, what should I be doing in the air, a lot of the time I'm doing AC Fair > ftilt.
Depends where your opponent is. Against an aerial opponent, I'm a strong advocate for using Fair in most situations because it's absolutely massive and sets up for good pressure afterwards. Against a grounded opponent, AC Fair is fine as long as you feel like you won't get shield grabbed. Keep in mind that Ftilt rarely kills in this game, so AC Fair > Jab or Grab is by far a superior and safer option. Both come out faster then Ftilt, and cover more options. Hell, even AC Fair > Dtilt is better then Ftilt in this game. Shield push on Dtilt is really strong and Dtilt actually kills.
I'm also against using Nair because you probably could have done something better like Fair in nearly every situation. Fair is huge. Nair also sucks in general.
Should I be using Up B OoS?
A big big YES. UpB got heavily nerfed as a defensive tool in this game, but it is still one the absolute best reversals out there. The loss of invincibility means you just have to time your fortress better. There are still many guaranteed punishes that you can get with a well timed Fortress that would have been impossible with a shieldgrab or any other shield drop option. The multi hits of the fortress mean that you're always at frame advantage after you hit someone with it, so it's great for resting positioning as well as applying pressure afterwards. Also, the small hits can sometimes trip, setting up for some big tech chases if you can react to it. (this is not a gimmick, this happens OFTEN) These reasons are why I still feel that default Fortress is the best Fortress. It still gives you a defensive tool that Bowser desperately needs in his gameplan.
Nair seems really good against people on platforms.
It's ok. I very much prefer Usmash in this game. Since people cannot slide off platforms, Usmash is insanely good against people on platforms, especially since people won't just slide off the platform and punish you anymore. The autoguard on Bowser's shell prevents people from dropping through the platform and hitting you as well. It just covers so many options. Even if you opponent blocks, you take off a huge chunk of their shield and still have good positioning. If you think they're gonna stay on the platform or try and drop through and hit you, keep Usmashing. If you think they're gonna run off the platform, try Utilting after blocked Usmash or simply blocking and Fortress OoS.
I like to shield camp and grab people, one of my main approaches is run > shield > grab things, I'm not sure if this is me being bad or #newmeta.
Sometimes the best approach is to just walk at people, because at least that way you can tilt and jab whenever you want. This was applicable in Brawl as well, since Bowser's out of run options were basically just Fortress and shield really. Running is still good for covering space of course.
Throws, which ones do you like most? I use a combination of f/b/d and ignore uthrow completely.
Obviously, Fthrow or Bthrow people offstage. Dthrow can be used instead of Fthrow if you want your opponent to be slightly lower vertically. If you know your opponent is fond of jumping early in their recoveries, this can help you follow up easier. Depends on the opponent and character really.
Uthrow is pretty bad all around, but has a place on Yoshi's or Battlefield. It guarantees huge positioning advantage because of the platforms. If your opponent is at a low percent and cannot be chucked offstage, consider Uthrowing them over the platforms to give yourself good positioning.
Finally, should I be edge guarding? Sometimes I get footstooled and die and this makes me VERY unhappy.
Yes I think you should be edge guarding. Don't get discouraged if you get gimped or get footstooled. That's how you learn. The only way you'll get comfortable going offstage with Bowser is to try ballsy things and get aggressive. Even if you die a lot, just remember what works and what doesn't. A good edgeguard can secure kills, and we all know how important that is in this game. (everyone lives forever because Vectoring is stupid)