We try to keep fireTick set to off on the server so you should be fine using it in your own decorations.
You are welcome to go into the exploration world Nether all you want for supplies, as that world gets reset a bit more often than the main worlds.
We used to have strict rules against mob farms, but now they are acceptable as long as they can be "turned off", and they aren't particularly massive. Turned off, just means they are lit properly enough to not spawn, unless actively being farmed. Massive GrinderBoxes (NOPE cakes) are STILL not allowed. But a bit of a trap on a single dungeon can be fine, as long as you are willing to share it with the community. Keep in mind that zombies and other undead react differently in water, 1.13 and up, which makes drowning traps far less practical these days.