some horses will barely earn anything in careers so yes it's probably best to trial them and see how well they do! sometimes horses just can't perform well in anything (halter, discipline, career) so using them for explore is best for them. and generally, haltering 1 and 2 star horses doesn't give much in terms of money or exp anyway.

I should also say that breeding two 5* horses doesnt always mean you will get a 5* offspring. Each breeds BV varies and its a secret up for players to discover.  This spreadsheet has very helpful information as well as places to keep track of your horses on if you want to make a copy for yourself. This one is for figuring out your horses best discipline and this is for careers. I for the life of me can't remember who made them all, they are somewhere buried in the forums, but figured I would share them here to find easily. 

We always recommend newer players starting with careers as they cost nothing and always generate income. Levels will come the more you train and use them. And its always good to have explore horses and try to sell some stuff you find. Pricing can depend, espcially on wilds so its always good to ask for help if you need with pricing here or in Discord 

I have another question, haha. 
So I see that skills are gained through careers, but I also see that it says that skills are gained through halter shows. In what way are skills gained through halter shows? I see that my career horses have 3 specific skills trained up because of the model horse career. Do halter shows skill your horse across the board or specific skills like the careers do?

the skills horses gain from halter shows are random

Back again with another question.
Is it not good to level up your foals in halter shows? I noticed that many players don't seem to be running their foals in halter shows.

I always show my foals in halter until they turn 2. The only ones you cant are crossbreeds that arent a part of a campaign so not sure why one might not enter them but I get them started as soon as I can

Awesome, I was getting a little worried that I may have been doing something wrong. I figured it was a really good way to get them leveled up by the time they are breedable.

Yep, get them going as soon as you can 

i would personally halter foals daily even if they perform terribly and lose money, the xp gain is worth it to me and i can always gain back the lost money with a better working path once they're grown up.

What are your opinions about the employees? I am hesitant to buy more acres because I don't want an excuse to buy more horses before mine are ready to breed as I kind of estimated my current stock and I think I would like to breed a few of my horses before buying any more.