Madanm. It is getting somewhat confusing where to place the adjective in sentences. Pou egzanp: Yon tranzaksyon presi. oswa, Yon presi tranzaksyon. Se vreman bon. oubyen, Se bon vreman. What has "thrown" me is: "Gran fi yo ak gran gason yo." I know this is correct, but here the adjective is in front of the noun. Up above it is not. Are there specific rules in Kreyol for this, or is it by sound only? I believe the first examples above are correct, "Yon tranzaksyon presi." for example. Mesi anpil.

Only a handful of adjectives are placed before the noun in Creole.
They are not too many, so you can even memorize them.

A.
Some adjectives that describe size: gwo (big), gran (great, older), katafal (huge), manman (huge), potorik (brawny), ti (small), ti zing (tiny), ti kras (small).  ("laj" → large doesn't belong in this group)
egzanp:
yon gwo gason
yon manman fanm
yon ti zwazo
yon ti kras pen
Li te achte yon gwo manman kabrit.

B.
Some adjectives that describe order: premye (first), dènye (last), senkyèm (fifth)
premye bagay la
yon dènye fwa
Yon senkyèm machin te antre.

C.
Some adjectives that describe age: vye (old, shabby), [ansyen (ancient), jenn (young), nouvo (new) can go before or after at times]
yon jenn fi
yon vye kay.
yon ansyen legliz oubyen yon legliz ansyen


D.
Some adjectives that describes  state /condition : bèl (beautiful, pretty, nice), bon (good, compassionate), move (bad)
He gave me a beautiful ring.
Li te ban m yon bèl bag.

Ou konpoze yon move nimewo.
You've dialed the wrong number.

Li se yon bon moun.
He is a good person.

E.
And also some adjectives that describe quantity: anpil, kèk, enpe, etc...
____________________
Yes, you are correct about "TRANZAKSYON PRESI", "presi" comes after "tranzaksyon" because
The other adjectives come after the noun- ...If I were to pick any random ones:

1. I like Haitian bread.
    Mwen renmen pen Ayisyen. (adjective "Ayisyen" come after the noun "pen")

2. The police car was speeding.
    Machin polis la t'ap fè vitès. (adjective "polis" come after the noun "machin")

3. I like the blue dress.
    Mwen renmen rad ble a. (adjective "ble" come after the  noun "rad")

If you were to have two or more adjectives:

4. I like the beautiful blue dress. 
    Mwen renmen bèl rad ble a. (adjective "bèl" comes before the noun "rad" and adjective "ble" comes after the noun "rad")

5. She bought the small red car.
    Li te achte ti machin wouj la (adjective "ti" comes before "machin" and adjective "wouj" comes after "machin")

6. A great wonderful God rules over the whole universe.
    Yon gran Bondye mèveye ap reye sou linivè antye. (adjective "gran" belongs before and adjectives "mèveye" and "antye" belongs after the noun)

7. A beautiful attractive woman was asking for you a few minutes ago.
    Yon bèl fanm atiran t'ap mande pou ou sa gen kèk minit. ("bèl" belongs before and "atiran" belongs after the noun.)

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words