At the start of the game the limits are:
Boomer 3/5
Spitter 2/5
Smoker 3/5
Hunter 3/5
Charger 2/5
Jockey 3/5
That's 3 + 3 + 3 + 3 + 2 + 2 = 16 for a total of 10 players. For the most part this is fine and no-one ups the limits except on levels that need lots of chargers or spitters.
My suggestion is twofold: 1) Make it random which SI gets which limit, so it's not always spitter/charger that are 2/5 (obviously both teams would have to get the same random set on any given map, but it could change between chapters)
2) Change it so that instead of [3, 3, 3, 3, 2, 2], the limits come from a pool of [5, 3, 3, 2, 2, 1]
The total is still 16, but one of the SI you get in abundance, and another you only get 1 of (at least till the limit is upped)
I think this would make for some interesting games, and would also lead to the SI limit increase thing being more important early on.