About rushing cavalry (and please correct me if I'm wrong):
Upgrading a pikeman (30 shields) to a musketman (60 shields) costs 90 gold. With Leo that will be reduced to 45 gold. The rushing price is only dependant on the difference in shields. So upgrading with Leo costs 45 / (60 - 30) = 1,5 gold/shield. So upgrading a horseman to a cavalry will 1,5 * (80 - 30) = 75 gold.
For 75 gold we can cash-rush 75 / 4 = 19 shields and upgrade 50 shields. So it's far more efficient to build horsemen and upgrade them to cavalry then to (short)rush cavalry.
Now the plan:
When we have researched Military tradition we will be able to build horsemen when we:
1 don't know chivalry
2 don't have saltpeter connected
The first is easy enough: we don't research or accept in a trade chivalry
Wrt the second:
There are two sources of saltpeter in our culture: NE of Norwich and E of Bonampak. We pillage the Norwich saltpeter and then mine it again but don't road it (netto cost 1 commerce). We put 3 workers and a pikeman on the Bonampak saltpeter. At the beginning of each turn the 3 workers put a road on the saltpeter, we upgrade the horsemen to cavalry and set some high productive cities to build cavalry, then the pikeman pillages the road and the less productive cities are set to building horsemen. With research down to a single scientist and all the other scientists changed to taxmen we will make huge amounts of money. We'll have an enormous army in no time
Of course we have to quickly build caravels as well. And there's no upgrading for them, but luckily they only cost half the price of a cavalry.
So how does that sound for a plan?