If you get new staff you will get nuts if your zoo becomes bigger.
I have:
12 Caretakers 5 stars
28 keepers 5 stars
10 mechanics 5 stars
12 vets 4-5 stars (more than i need, i keep 4 for stricly resarching)
also 8 3 star security peole and
58 vendors with 2 stars
All 5 star employees are working fully efficient, some keepers with high workload.
That is for roughly 230-240 happy animals in 23 habitats.
Zoo rating is 5 stars in everything but customer happiness which is 4,5 stars.
Even now my staff-pathway is quite full and i build it very generously.
I startet to train them as soon as possible as high as possible.
(edit: ofc while managing your money, i needed to fire some people at an early stage too, because i overdone it.
And research and get all possible kinds of vendors, in the early days its a real help.)
Edit:
A little hint for an early, steady money boost:
Buy one or two small exhibits.
Put 4-6 Giant Tiger Land Snails into each.
Those reproduce very quickly and can be sold for a good pocket money.
Its not worth the time once your zoo is running, but selling 30 cheap snails every now and then for 8-10 k money was
a big help for my early, clumsy planet zoo days.