I will take 客 as an example:
Can 客 form a compound with 席, 娟, and all the other thousands of Japanese 漢字?
If I put 席客, but not 客席, will the meaning be different? Or will it just have no meaning? Or is it only a grammar error? Even if it's a grammar error, can the meaning still be understood?
How flexible is the formation of kanji compounds?
Technically speaking, kanji don't compound. The Japanese language is not made of kanji. It's made of words and and parts of words, many (but not all) of which can be written with kanji. When you put these words or parts of words together, you can often (but not always) write the resulting compound using kanji.
Still, let's talk about "kanji compounds" for the time being. It's a convenient metaphor for how words are put together in Japanese because kanji so often correspond to the words or parts of words that can be compounded. Below, I'll reproduce a description from Habein and Mathias' Decoding Kanji pp.25-27, which I believe is now out of print. It catalogues several common types of compound:
When two kanji are in a syntactic relationship:
a. Subject-predicate
地震（ジシン, earthquake): the "earth" (subject) "quakes" (predicate)b. Verb-object/locative
造船（ゾウセン、shipbuilding): "make" (verb) "boat" (object) 在日（ザイニチ、(someone) stays in Japan): "to be" (verb) "Japan" (locative)c. Adjective/noun-noun
高級（コウキュウ, high class): "high" (adjective) "class" (noun) 国宝（コクホウ, national treasure): "nation" (noun) "treasure" (noun)d. Adverbial first kanji
不正（フセイ, injustice): "not" (adverb) "just" (adjective) 未定（ミテイ, undecided): "not yet" (adverb) "decide" (verb)e. Auxiliary kanji
私的（シテキ、private): "I/me" (noun) "-ic, -ous" (auxiliary) 突然（トツゼン, suddenly): "sudden" (adjective) "state (of things)" (auxiliary)When two kanji are in a nonsyntactic relationship:
a. Reduplication of kanji
方々（ホウボウ、everywhere): two of "direction" 月々（つきづき、every month): two of "month"b. Combination of two kanji of similar meaning
平和（ヘイワ、peace): "calm" and "harmony" 生活（セイカツ, living): "live" and "liveliness"c. Combination of two kanji with contrastive meanings
子孫（シソン、descendants): "child" and "grandchild" 左右（サユウ、left and right): "left" and "right"Three-kanji compounds are usually made with a prefix and a two-kanji compound, e.g. 新時代（シン・ジダイ, new era): "new" and "age", or a two-kanji compound and a suffix, e.g. 仕事中（シゴト・チュウ, while working): "work" and "in the middle of". There are occasionally three-kanji compounds where all the kanji have contrasting meanings, such as 大中小（ダイ・チュウ・ショウ, large-medium-small): "large", "medium", and "small".
This description isn't exhaustive, and there are other ways to catalogue compounds, but it should give you at least an overview of how they're put together.
Still, although native speakers make new compounds fairly often, learners are generally taught not to make their own kanji compounds. Instead, we're taught that we should learn as many pre-existing Japanese compounds as possible. If you create your own without a good understanding of Japanese, you have a good chance of not being understood―and if you are understood, you have a good chance of sounding quite strange.
Think about it this way: if you didn't know the compound television, you probably wouldn't come up with it yourself by putting tele and vision together. That's a word you've got to learn, and coming up with your own compound for it simply won't work.
There are a couple ways you can put kanji together you'll need to be familiar with:
What Habein and Mathias refer to above as "auxiliary" I would categorize under "prefixes and suffixes". There are a number of these you should know, such as their examples of 〜的 and 〜然.
Japanese counters such as 〜人 or 〜枚 which attach to numerals to form words like 3人. (These could be considered a type of suffix as well.)
But putting 客 together with 席 or 娟 (what is this? I have no idea) is the wrong approach. Until you're comfortable with Japanese, you should try to use the words that are already part of the language rather than forming your own.
