You can consolidate regiments if your manpower runs out/low, as reinforcing is expensive (basically reorganizes them). If you hover over the manpower at the top left you will see the how many men you need to reinforce the units, and how many you generate each month.
Generally if you do need mercs, it's best to get only infantry, try to keep your cavalry and artillery as normal regiments.
You should also try your best to keep up in military tech, it's very important indeed. And pay attention to unit composition. Early game you want primarily infantry and some cav. Mid game throw in some artillery as well. Late game artillery is king basically, and cav less so. A late game 40 stack might look something like 20 inf / 2 cav / 18 artillery. Also pay attention to attrition and supply limits, if your units have skulls on them they are taking attrition. You will always take attrition in enemy territory, but it's worse in things like Russian winters in Siberia, and in provinces with low development where supply limits are low (you can see what they are by selecting an army and hovering over a province).
I quite often find myself splitting my army (shortcut S key) to avoid massive attrition from supply limits, especially early game. Late game tech reduces supply limit impact and improves it besides.