If you don't know the transivity of a verb, is it always grammatically correct to use -ig- and -iĝ- to ensure the transivity is the one you're trying to produce? Or in cases where it's redundant, would that be incorrect (like a verb is already transitive, and you add -ig-)?

In some cases all three forms have a distinct meaning, for example from the Verbumado course on Lernu!:

interesi

Ĉevaloj interesas min.
Vi interesigis min pri ĉevaloj.
Mi interesiĝas pri ĉevaloj.

It might seem a good survival tactic, because you should succeed in getting your meaning across, and be wrong only half the time. Let's take as an example the verb boli. Suppose you've forgotten whether it's transitive or intransitive. Then (using your rule) 'I boiled the water' would be Mi boligis la akvon (correct). 'The water boiled' would be La akvo boliĝis (wrong, but understandable).
But this only works if you have learned the meaning of the root accurately, and if you have done that it won't be too difficult to understand whether it's transitive. So you might as well learn the full meaning with the transitivity!

I would not add ig or iĝ by default, even if I am uncertain. When in doubt, I have a general hierarchy I use. If I have no idea, I first assume that it is the same as the transitivity in English. If the English verb can go either way (a common situation), I try to think of the context where I normally hear that verb, and how it is used. For example, you will often hear "[something] rompiĝis", so I would take this memory and assume that rompi is transitive. Third choice, if I have no idea, I make a guess. In almost all cases you will be understood even if you choose wrong, and hopefully whoever you are talking to can correct you, particularly if you mention to them that you are not sure of the verb's transitivity. Don't be ashamed to ask, even experienced speakers will sometimes ask if they are unsure (I certainly do!).

eb.eric:
Vi interesigis min pri ĉevaloj.

Excuse me.
I'm a beginner and am confused about your usage of the preposition "pri". How would its omission effect the meaning of the sentence?
As in,
"Vi interesigis min ĉevaloj."
Or,with the addition of an -n.
"Vi interesigis min ĉevalojn."

I would very much appreciate any help.
Thank you.

You have to remember that sometimes the word order doesn't matter in Esperanto. If there are no prepositions, the noun without -n is the subject, the noun with -n is the object (if applicable).

*"Vi interesigis min ĉevaloj."
Here neither vi nor cxevaloj has the accusative, and since there is no preposition, they are both the subject of the sentence, and that makes no sense.

*"Vi interesigis min ĉevalojn."
This sentence doesn't make sense either, because the object of the sentence is "me horses". This sentences is saying You made me horses interested. And not "me horses" as in a colloquial version of "my horses". It's just plain wrong I believe.

"Vi interesigis min pri cxevaloj."
You made me interested in horses.
This sentence has the subject (-o), the object (-on) and a descriptive prepositional phrase. We know that cxevaloj isn't the subject because it follows a preposition (her e word order does matter). However, the whole phrase could be moved, as long as the noun still follows the preposition.

"Pri cxevaloj min interesigis vi."
Should be equally correct.

Correct me if I'm wrong!