We don't have to worry much, for with as so small an amount as is in his possession he won't go far and we'll eventually catch him.
I've made up this sentence and I'd like to know if the "for with as so" and the "as is in" parts are okay? Didn't I break any English rules here?
This sentence is a lot more complicated than it needs to be: that makes it hard for the writer to be sure that it is correct, and even harder for the reader to understand.
If you absolutely need to write something as complicated as this (and IMHO you don't need to here), it's generally a good idea to use the least ambiguous equivalant way of expressing each concept, for example for has two meanings (conjunction because and preposition intended for), so it is better to use because in a complex sentence like this.
We don't have to worry much, because with as so small an amount as is in his possession he won't go far and we'll eventually catch him.
your original sentence is almost correct, but you can't use an adverb of degree after the first as in an as..as construct because as..as is a kind of comparative. For example
as bald as a coot - ok
as very bald as a coot - not ok
Eliminating so and applying more conventional punctuation, we get:
We don't have to worry much because, with as small an amount as is in his possession, he won't go far and we'll eventually catch him.
Alternatively, you can keep so and eliminate the as.. as construction:
We don't have to worry much because, with so small an amount [of money] in his possession, he won't go far and we'll eventually catch him.
Better still, though, would be to cut out some of the entangled dead wood:
We needn't worry: we will eventually catch him because he won't get far with the amount [of money] he has.
We don't have to worry much, for with as so small an amount as is in his possession he won't go far and we'll eventually catch him.
Grammar errors aside, I want to help you write in a way that is clear and easy to understand.
Use short sentences. Make them simple and to the point. Keep related topics together.
The very beginning and the very end of your sentence are one thought: "We don't have to worry much…we'll eventually catch him." The part in the middle is the reason behind that thought: "with as so small an amount as is in his possession he won't go far."
So let's take the beginning and end and make it one sentence. The reasoning in the middle will be a second sentence. (I assume the "amount" you're talking about is money.) And we'll correct the grammar and tighten up the wording:
Don't worry, we'll catch him. He doesn't have much money and won't get far.
