Hi
Well, i guess most of your questions have a "it depends" kind of answer

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A very basic rule of thumb i use on AGEOD games that usually gives good results is "try to do things in a way that they would have worked in real life". IMHO on AGEOD games you can do without much number crunching and use instead common sense and knowledge of historic tactics and strategies as a base. They try to simulate XVIII century warfare and they succeed in most. So what worked back then, use to work in WIA.
I'll try to answer your questions...
When should a unit attempt a forced march?
Obviously, when you are on a hurry to arrive somewhere
In game terms i use it mostly when a stack planned move takes 31-35 days, so, if successful, i will end my movement under 30 days and arrive at my objective in just one turn.
The % success change depends of the unit type/quality. Usually militia/wagons and the like won't make it. Use it mostly with light forces that have high % chance of archiving it.
What are the optimal ROE settings for a small force? Should a small force avoid combat when moving?
It completely depends of the situation. If you risk finding bigger and stronger forces on you way, evade. if your force can be expected to catch some weak force, don't evade.
In general, move on defensive as it costs you less cohesion than on offensive and it's safer if you stumble on an enemy force (defensive can use terrain to their advantage). On very safe areas (no enemies around) you cna move on passive for still less cohesion loss.
Should Indian units always have their ROE setting for ambush?
Ambush only work for stationary units. So if you are moving around it won't do anything. Its really tricky to manage an ambush.
If you don't have a specific task for a force, is it better to remain "inside" a structure? Or outside a structure, so it can retreat if attacked by a superior force?
Again it depends. I use to leave small garrisons inside (safer against small raids) but bigger field armies and irregular forces outside so they don't get trapped.
Of course, having a fort or fortress makes it more interesting to stay inside, but be prepared for a siege or a "do or die" battle with a force inside a structure if a strong enough enemy force attacks.
How do you calculate the relative strength of an enemy unit - especially with fog of war = ON?
Tooltips will give you different levels of info depending of your detection value. Use Indians, spies or irregulars on adjacent regions (and on evade combat!) to have better info.
The little colored balls on the base of the stack will give you a general info about size (number of balls) and health (color of balls). With the supply filter on, the color will reveal their supply status (green good/red bad).
When marching a force to contact with the enemy, is it better to move your force short distances (maybe just 1-2 adjacent land areas) in order to maintain cohesion?
Again it depends. Moving reduces cohesion and combat power. But if you are in a hurry... Sometimes its better to arrive sooner but weaker than later but stronger... others don't
In general, let the guys rest form time to time or move in 15-20 days long movement so they are fit enough for a battle.
Is it wasteful to have a force with two (2) supply wagons?
Each supply units carry a fixed supply amount that you force will consume. So having 2, 3 or more will give them more turns without starving or the need to resupply. So again it depends of the force size, your number or wagons and the force mission.
Is it "safe" for Leaders to travel alone? Or should Leaders always be stacked with combat units?
Yes, its pretty safe. They have good evasion rating so they can cross enemy controlled and occupied regions with little chance of capture/death (like 95% chance or so). remember to always put them on evade combat.
I have them stacked with troops except when i want them to travel far away to maybe lead another force in a different colony. They move much faster alone.
Hope it helps!