Here’s the thing with both of these points that you brought up:
It’s still only one slot that this would occupy. So while it would be used likely to adapt to map or other situational variable, it wouldn’t become overpowered. It would allow people to bring something that SPECIALIZES rather than just something that is outright OP in MOST situations.
Currently we face a huge issue with people taking endless squads of assaulters, flamethrowers, paratroopers, etc right? Now imagine if instead of just taking those, there were an option to take certain other classes because you know of specific threats or opportunities that occur in certain maps. AT squads, mortarmen, engineer squads could now have a lot more utility and help break up the meta of the game, simply due to be used on maps that they are actually needed on!
As far as players that pay for slots go, the initial couple of slots are VERY cheap. Considering that the game is free to play, but at the end of the day its still work for the devs, it needs to make money somehow. EVERYONE is hurting for cash right now, the devs included. I see no difference between paying the devs for a bit more entertainment and paying for the convenience of fast food. After all, its about the same price for extra slots.
Does there need to be some major reworks on balance to prevent spam squads from being so effective? Absolutely.
Would a situational squad slot ADD to the spam squad issue? NO.