Learn English – “that is” or “that are”

In the case of and, it's very simple. Regardless of whether the nouns are singular or plural, joining them with and "creates" plurality.

The cat(s) and the dog(s) are under the bed.

Again, whether cat and dog are individually singular or plural will have no effect on the verb. The verb will consistently be plural following nouns joined by and.

Or is slightly more complex. Two nouns joined by or do not create plurality. Instead, the verb will be made singular or plural by the noun closer to the verb.

The cat or the dog is under the bed.

In this first sentence, "dog" is singular and therefore we use the singular verb form "is."

The cats or the dog is under the bed.

In this second sentence, you can see that cats is now plural, but the verb is unaffected. The verb form depends entirely on dog because it is the noun closer to the verb.

The cat or the dogs are under the bed.

For this third sentence, now that dogs is plural, the verb changes to the plural form are.

Check here for a guide on subject-verb agreement, including what I covered about and and or.

"All that combined" is the right one. However,

If "all that combined" is used without a preposition "with", it doesn't serve the purpose of using the word "combined".

If you have to use the word "combined', I would suggest using it as follows,

All that combined with a desire to find the best suitable college for my dream, I decided to visit a few colleges to get a professional advice from professors and students.

It is just a suggestion because using only "all that combined" as a conjunctive adverb, it sounds a bit awkward.

Hope it helps.