1. Is it safe to assume all new DLC factions will be added sequentially? I've started adding my units/weapons/ammo/etc... to the very end of the lists (i.e. starting with unit ID 5000 and moving down) with the theory that I can at least copy and paste my XML entries into new copies of the base DB if needed. My worry is with the IDs hard set, if I don't place my stuff in a safe place, I'll never be able to merge things when a new DLC faction is released.
Our database designer would be better at answering this, but yes, I believe the units etc. will be added mostly sequentially. I think it will take a considerable time before the official database will cover several thousands of units. Perhaps at that point there will be better tools for moving the database items around.
2. Is there a way to assign different sprite scales to "unknown" units. My issue is I'm using a bunch of sprites at a smaller sprite scale (6.7). They look fine, if blurry in game, but the issue is you can always tell when it's one of my units coming because the 'unknown tank' marker is ridiculously big. It's getting scaled at the same factor as the vehicle's main sprite (i.e. the default which I'm assuming needs a scale of 25.6 like all the default vehicle sprites to look normal) and I can't find a way to either change it or substitute a smaller scaled copy I made for that unit.
I think currently there's no way to adjust the unknown sprite scale separately. One way to 'fix' this would be up-scaling your sprite images to use 25.6 pixels. I don't remember anyone mentioning this issue before so the scaling system hasn't been improved.
3. Is there any hope of some sort of DB merge feature or tool in the future? Like, take a batch of units, or a faction and be able to transfer it and it's dependencies into another DB. The modding tools are powerful, but I'm worried things will get a bit siloed if there's no way to combine them. Like let's say modder A makes an awesome China faction, and modder B makes an awesome Japan faction. How can I get them to fight each other? I likely can't merge them manually, since modder A and modder B didn't coordinate and they've used the same hardcoded unit/weapon ID numbers. What about some sort of batch copy/paste within the tool so you could at least shift things around to free up ID numbers (i.e. if you could shift select like, a dozen units and cut and paste them elsewhere. That'd be extremely helpful).
Yes, we can try to implement something like this in the future.
4. What do the vehicle size parameters (i.e. turret/hull length, width height) influence? The likelihood of a turret vs. hull hit? It can be tricky to find for some vehicles so I've just been ballparking it, but will it work fine then as long as I get the ratio of hull size to turret size close?
In the manual chapter 5.1.13. there's information about the vehicle size:
"Size affects how easy the unit is to hit and see. Large vehicles make more dust when they move. The larger the unit is the bigger explosion it can make when it explodes.
The ground vehicle and pillbox size is based on the hull and turret dimensions. If a vehicle is dug-in or in defilade then only the turret is visible. If the vehicle has no turret, then 1/3 of the hull is visible. The size is compared to the size of an ‘average’ sized vehicle, which is the size of a T-72 tank. Based on this, the vehicle is considered smaller or larger than average."