I think rebellions still trigger the same. They have the same number of units too I believe... rebellions always seem nastier on huge... so effectively you halve the size of the rebellions by playing with smaller unit sizes.

Don't use rubbish units to garrison. Use spears or crossbows, as they get outdated slowly swap them into your attacking army and let them be used up or just get rid of them. Another thing to look at is that it is not the cost of creating the unit but its upkeep you should worry about. As such some of the more expensive units are more cost effective (Saracens cost as much as humble spearmen).