After now having played several games with my local group, I can provide a little feedback on the topics of units having rules about outnumbering and having bonuses for increased numbers.
Outnumbering works rather well without having to do really anything to the rules. If you have enough models within melee range to outnumber the opposing models, then you get to use the outnumber ability. The important thing is to keep in mind melee range on nearby models. You might think you outnumber 2-1 with a couple stormvermin, but there could be another model with a 2-3" reach just a little ways back that evens up the odds.
As for increased regiment size abilities, we played around with a few variations on this and this was the one that worked best for us.
For units that have special bonuses for regiment size, the number of units required to utilize those bonuses is divided by 5 and the bonus is only in effect while all of those models have no more than a 1" space between any two of them.
So for example, Seraphon Saurus Warriors Ordered Cohort now reads as
This seemed to be a good balance between allowing these units to use their regiment size abilities in a smaller scale, but still requiring a reasonable number of your 15 model limit to utilize it.