1. Have a heavy weapon in your list early since it'll be tough to add one of these in the campaign without significant investment.
It's been a while since I played Necromunda last, but I was always rather divided about heavy weapons; special weapons might be slightly less firepower, but they're a lot more mobile and considerably cheaper (and much more feasible to replace if you lose them).
And if we're proposing heavy weapons as vital, several Shadow War factions are in trouble, not having access to any at all.
The question isn't about the specific weapons though, but about a campaign strategy when *all* of your models are too expensive to buy without spending Caches, getting the right Subplots or having the right Guerilla skill (that is to say: when even the cheapest recruits are over 100 points after being given an actual weapon).
Trying to acquire the Scavenger skill would seem to be a fairly high priority for teams like this (being able to recruit 150 pt models would completely change things), but until you have that, any losses will be difficult to replace.
2. You start bottling once you lose 25% of your models. If you start with six, that means once one of your Sisters is down/out of action, you'll start taking bottle tests. Therefore, I'd advise sticking to odd numbers greater than 3 to give you one model leeway before bottle tests: 5, 7 or 9 models +
You've got the maths wrong. A quarter of six would be one and a half, which would be two models down (because the rules specifically say to round up). The threshold for being able to brave a model down starts at 5 and increases by one for every extra four fighters. (Which for most factions means either 5 or 9).
Having six models though means that should one kick the bucket, it wouldn't be a desperate priority to replace them - even if reduced to a five model team, they'd still need to have two models down to start bottling.