Mellifluous Magenta
Anytime you look at the Codex the first rule of thumb to remember is to be
reasonable in your submissions for any category. If you come into the Codex with that frame of mind you'll be just dandy. That being said...If for
any reason I suspect someone is trying to take advantage of a loophole, unwritten or otherwise, that submission
will be denied. Gaming the system shouldn't be the goal over creating vibrant, new, and interesting content.
We don't have a banned species list at this time, so you don't need to worry about that. Your question isn't easily answered though. Often times, species are custom created and have no "base line" to compare it to. Take the Bryn'adûl for example. There were endless of their numbers, but, they were quite strong and powerful. What sold them for the most part was how well individual writers wrote them. Yes, some submissions required a second look but it was overall very well maintained.
We don't have the aforementioned factory rule in place in
every area of the Codex because we judge each submission individually unless there are several submissions deliberately designed to become a greater whole. We don't necessarily seek to judge for balance but we will step in if needed. Especially, if the submission doesn't hold to the general idea of Star Wars or runs afoul of one of the rules. You will note that we often warn submitters that a particularly powerful species/npc could be easily reported. If that happens it will be pulled and thoroughly reviewed until it is brought back to standard expectations.
To that end an "overpowered" submission could exist, however, if the writer uses it wisely they won't have any trouble. Hence, why we give the warning but "may" let something pass. It's give and take.
There are actually already restrictions in place for NPC Units when it comes to sizing, force-sensitives, etc... That can be found
here and especially in the combat unit template, table, and description.
When it comes to a species? Not so much leeway as you've described, no. We judge each submission individually based on its own merit.
When it comes to NPC Units, yes, and that can be observed in the link I gave above.
(Just one Example: Combat Units containing a majority of Force Users are not allowed to be submitted at Large Size or Common Availability. Please keep in mind that the more powerful a unit is overall, especially in regards to the Force, the smaller and less available it ought to be. Be sensible and reasonable.)
If there is a particular example you can give me I'd be happy to discuss it.