I'm using SM now to get a feel for it. But I would like to know how you get a split stone thrower to recombine? In fact I've split a ST and then 20 turns later spilt one of the original split ST units again! And all still have the 0.66 str.

But I can not recombine any of them! So the only use I can see is for game start faster scouting and not much else.
To merge, you must bring 3 units of the same type to the same tile, have all 3 be healthy, none be grouped, none having moved this round, all be the same unit and group volume. At that time, on any of their control panels you'll see a 3 ring mesh action symbol for merge. Select that symbol and it will popup for you to select the other two units you are merging with.
You can't split an already split stone thrower since that brings their group volume down to solo. (If you're playing with Size Matters Uncut and are able to do that then there might be a bug with SM Uncut.) That's strange and not something I've seen possible. Is it possible that you had trained another stone thrower and split that one?
The splitting aspect is strategically useful in three ways:
1) For early scouting, yes. Wanderers are solo units so cannot further split and as explorers they cannot merge or split anyhow but it does help for that first stone thrower unit to split them up and send each individual on his merry way - likely to his death eventually but he'll manage to get some land revealed first and the survivability is not much less than that of the party sized group.
2) Splitting defenders to cost attackers more attacks to take a plot.
3) Splitting combat units and then giving them bottleneck promotions so they can take full combat advantage of their lower group volume and more numerous unit count. This can work with surrounding. Very tricky to make this beneficial though because the lower strength is really disadvantaged. On the vanilla combat engine this option would probably be more valuable.
Usually on SM, you'll want to merge your military units to obtain greater strength. But criminals, ruffians, law enforcement, healers, strike teams, none of these can split or merge. So it's units like swords and archers that can become truly massive.
You're also limited to being able to split or merge as many times as the count of the eras you're in, Prehistoric counting as one, then you can split or merge one more category further in Ancient and so on. At a point, you won't be able to build enough units to have them merge to the max so it becomes a real judgement call whether you have enough in your primary spearhead units, understanding as well that very small group sizes like heroes can be very powerful against these giant forces if they are developed with the bottleneck promos. Of course, highly merged units can also take advantage of the swarm promotionline to counter bottleneck completely, but then those units usually aren't so great against equally matched large forces either.
Thanks for trying it btw. If nothing else just so you can see how it blends with other mod elements will be helpful for you as a team member. And I think it shows some personal growth

I think both of us have experienced a lot of that in the last year as a result of our teamwork.
A while back I had a self-imposed rule of 1 Law Enforcer per 10 pop...it worked great...kept it a real challenge with Crime at times peaking in certain cities because of the nature of the city.
Now it seems I need almost 1 Law Enforcer per every 1 pop....
Law Enforcement units are not as effective as they were because they cannot develop their anti-crime promos out to the max any further, nor do they get a free one to start, and the earliest ones don't have as high a crime fighting base as the old Town Watch used to, but they do come into play earlier. You will probably need more of them, yes, but what I've been finding is that anti crime buildings are built too late to avoid sparking the booster rockets of crime that criminal spawns can become if you build all production buildings first. You really have to blend in some anti crime building efforts throughout the process of building up your production and food or it's too late and the city has taken off with crime. If you let this happen, you'll find it can take upwards of 15 Enforcers to take care of a problem like you have there. This gets a little easier later in the game as LE units expand in their capacity to fight crime with more base, more promos, more buildup stages they can reach with fighting crime.
I also noticed that the little bit of added crime from banditry is a huge problem for your first cities, which is good. I believe we now have made it so that if you build the bandit's hideout, you can maintain it while you switch off banditry so when you're on banditry, unless you're very good at fighting crime, get off it as soon as you can, especially for the benefit of your newly planted cities.
For a complete breakdown of the criminal spawn mechanism:
This is the latest explanation, written as an addendum to the v37 Player's Guide. Should be VERY informative.
You also talked about the animals. I've been considering reducing spawn rates a bit further still but some of the latest tweaks seem to be getting a better balance of spawns at least. I'm also not sure that it's beneficial to the game to let them be without maintenance costs because it's allowing me to hoard so many it's getting ridiculous. I thought I'd like that but I'm not so sure now. I like being able to get a lot of hunting in but it might be nicer to not be able to hold onto SO many. Certainly better for the processer than to have hundreds of animals build up in your cities.