Yes, companions are huge money makers. I've been collecting as many epic and rare ones that I come across and that my resources allow. The ones in the MM might seem expensive initially, but they pay for themselves over time. Currently I'm making 21k-35kish pebbles per 6hr playtime through Nana, and that's minus the fee and I've only started really collecting them like 1-2 weeks ago. I've set myself goals like collecting enough Kapros and Rockbreaker Ox to one day bring in 100kp per 6hr play time and it keeps me motivated ^^
On top of that I recommend combining tasks when you do things like exploring. If you explore in the campaign for something like finding WWs, Rockbreaker Ox, Kapros, lvl up companions, etc. use that time to lvl up fodder as well. Even if it's only for one extra wolf and you don't get to lvl 10 before releasing it, it's still those extra pebbles that add up really fast. On top of that, selling the trinkets from that same run can bring in thousands/hundreds of thousands of pebbles depending on how long you go far and what you bring in. All of that adds up really fast.
Also, things like swords, armor pieces, stacks of food, metal bars all sell in the FM. I only started selling about a week ago and it's brought in hundreds of thousands of pebbles.
Edit: There's a lot of other companions that bring in decent pebbles too, but the ones I mentioned are the main ones I've been focusing on.