I understand the convenience of fem'd ceeds, especially if you're facing strict LEGAL plant counts, but just as a suggestion here,
if you have the room to flower out 3-5 full sized plants at once, you almost certainly have the room to sex them if you were interested in doing so.
You don't need a whole "room" to veg and sex plants, nor does your veg area need to be in a different room than your flowering one, so long as you make it light-tight, and only open it up when the flowering area is lighted too (which you would anyway).
In fact, since veg plants are necessarily smaller than flowering ones, veg areas are usually considerably smaller than corresponding flowering ones.
If you're interested in doing so, you can create and maintain a perfectly viable seedling/veg/clone/mothering area under cheap tube or CFL fluorescent lights in a tiny cabinet, bookshelf sized area, or even a smaller one. In fact, you could probably sex and veg three plants in a PC case, and/or root half a dozen clones in the same space if you wanted.
I happen to use 2 liter containers for this, but you could potentially use smaller ones if you liked. Clones can be rooted and maintained for long periods of time in ordinary plastic cups. See here for a complete overview of my complex cloning "system", plus a look at a plant in veg, pre-flower. You can scroll to the end of the report to see how the plant progressed from small container to large one through flower:
https://www.rollitup.org/grow-journals/585422-sickmeds-williams-wonder-grow-journal-2.html#post8396492
Now, even though having a small seperate veg area is a little more work, it gives you a LOT of extra flexibility in terms of having a place to start ceeds, maintain clones, or do some vegging. Its also a *LOT* more productive than running serial 24-0 then 12-12 hour veg/flower in the same space. The reason is that the larger main flowering area is ALWAYS being used to flower. . .there is no "down" time like you get doing "runs" of 24-0 veg. As soon as you harvest one plant, you replace its space with an already waiting clone or vegged plant from the veg area, so you're not wasting any flowering space either.
Also for your consideration, if you do choose to run a continuous 12-12 flowering area, its also perfectly viable to sex plants in there if you like.
Again, you can just keep the plants in small containers to save space until they show gender. So long as you are dilligent and check the pre-sexed plants on a regular basis, you'll have plenty of time to cull males before they actually make pollen to ceed your crop. Now, running 12-12 from ceed may not be yield-optimal for some strains because you don't have the same time to train them or let them get super big before flowering, but it actually does work well for stretchy/long flowering pure sativa or sativa-dominant dominant strains, because its another way of controlling their height.