I ended up playing this mission twice because I was an idiot and missed one of the + level objectives early in the mission.
So I can confirm that you always get your army "topped off" to the appropriate level at the reinforcement spots, no matter how many of your units you've managed to lose in the meantime. It does not change with game difficulty.
- Before 1st boss; 6 Roaches
- Before 2nd boss; 6 Roaches + 4 Hydralisks
- Before 3rd boss; 6 Roaches + 4 Hydralisks + Ultralisk
Although I admit I have no idea where the reinforcement spots are in the 3rd part, because I made it all the way through without losing any units, mostly by leaving them behind and soloing most of the area with Kerrigan.
I assume there's one right before the boss though, like there is in the first two parts. I know that you do get refilled (to 6R+4H+1U) before the last boss though, because the 3rd boss's constant spawned Banelings are quite the hazard to your normal units.