Neither version of your final sentence is particularly idiomatic.
"mightn’t" isn't often used in English. Choose "might not".
The meaning of "might" is to add an element of conditionality or uncertainty. This could best be accomplished as follows:
But not all of them will necessarily work for you.
Other options to be aware of, although I don't think they are precisely right for this case:
But some might not work for you
But they might not all work for you.
When editing text you have to consider the context: what are the previous sentences, and what are the next few sentences? You have provided a part of the puzzle (the preceding text) but not the next part (the following text). This makes it difficult to give a conclusive answer.
Should I use "those" in every case, except if I'm indicating a group of persons?
Here are three choices, any of which are possible:
"They" can be used with inanimate objects. From wiktionary:
"I have a car and a truck, but they are both broken."