This is giving me echoes of what happened to Chaos Reborn at launch, with their user-created realms being not-well designed, released un-vetted and alienating a lot of new players. However, cannot conclude until I've actually played the beta.
I hope your floor generation algorithm has a system that will rank room/enemy difficulty and control the distribution of challenges. 
Also posting to confirm that you're sending a short design guideline for esp. room content submitters (volunteer and contractor), who will have hopefully played the game a bit as well.
Shouldnt be a problem.
This gets tested alot, and problems tend to be fixed quickly. Additionally, anything submitted by anyone is going to have a seperate name attached to the file that contains the XML code or the room data, depending on which it is. Makes it easy for us to keep track of submitted content.
As for the difficulty control, ah, that's not quite ready yet. Right now enemies are not entirely gated by floor like they should be (because I havent done it yet), though, new ones are being created for certain floors only, now that some of the overall balance of the game and the player's ship overall is starting to appear.
Some enemies are also mis-categorized. The rooms do not have enemies that are specifically set, or rated by difficulty. What's done instead is that enemy categories are chosen and placed within those rooms, the game then chooses an enemy randomly from that category pool every time it loads the room. The trouble is figuring out what category each enemy goes in to begin with (this is typically the job of whoever made the enemy, though for submitted ones, either myself or Chris are likely to set that ourselves, probably). I have found this difficult at times. The more complicated the enemy, the harder it is to do. And as a rule, everything I make is complicated. I'm not good at easy/simple, as anyone that knows me is probably aware of.
Now, when it comes to enemies specifically, you can also send them to me to have a look at, since part of my job is to keep the blasted things as balanced as possible. Often, submitted enemies will need some tweaks or even additions/subtractions to their overall pattern.
Right now, my priority is making more bosses, so any normal enemies that anyone can come up with are appreciated. Not looking for super-complex ones; if it's complicated as all hell and tough as nails, well... that's my own realm of design and there'll be enough of that in the game already. What I'm NOT good at is making easier foes yet at the same time keeping them INTERESTING. When I try to make something interesting, it becomes a screaming murder-ball. Early or mid-game enemies are needed, as are enemies in low-power categories.
Though, things like rooms are just as important too; the more rooms in the game, the more variety there will be in exploration, and that's ALWAYS good. If we can have a huge number of rooms, that'll really help alot. In addition, rooms are the easiest to make. The room editor is nice and simple.
And again, these rooms are going to be looked at and tested before being stuck in.
Generally, if things dont meet the standards or are wildly imbalanced or messed up, they probably wont be used. And it's not hard to figure out when this is the case.
The important thing is NOT TO RUSH when designing stuff. Take your time, and test the hell out of it, regardless of what "it" is at the time.
And rule #1 to me, when it comes to design: If *you* dont think it's fun, others likely wont either, so make sure that you enjoy it before you submit it (this doesnt guarantee it'll be used, but it sure as heck helps a ton!). When this rule is broken is when things go all wonky.
