I mix the fishmeal with my feed for the chicks with no problems.
Check the last reply below for ratios.
As Linda said below, I'd wait a bit on the whole BOSS (Semilla de Girasol). That said, my feed is very coarsely ground with a lot of whole seeds, yet I start feeding it by the end of the first week as FF. If they were with a mother hen foraging, they'd be eating similar size things. Hence the importance of grit, unless it's available naturally on range.
There's really 4 sizes of grit. 1,2 and 3 for chickens and 4 for adult turkeys.
http://www.tccmaterials.com/pdf/CSpoultrygritdata.pdf
Grit is a good idea regardless of what you're feeding. It helps develop the gizzard and therefor the health of the chicken down the road. I give grit right out of the hatcher. I sprinkle feed on the paper towels and sprinkle the #1 grit on top. After a week I put it in a separate container from the feed and continue that all through their life.
The Spanish word for grit is gránulos.
Perhaps if all the farmers there free range, they don't provide it.
If you need other Spanish words for anything feed or animal related, I can help.
I wouldn't ferment live animals.
The chickens love hunting for them in their bedding to relieve boredom in winter.
It's also between 60 and 66% protein so it takes very little to bump the total.
I use a 16% coarsely ground organic grower for all the birds. For chicks I mix in fishmeal at a 10:1 ratio and that bumps the total protein to 20%.
Here's an ap that will help you determine ratios to achieve the desired protein.
https://homesteadapps.com/app/free/feedcalc/pearsonsquare.php