I have a bottle of liquid, is that not what you should use vs the paste?
If possible you should get the pills made for humans. But the medicine it the same. The mgs of Ivermectin per kilogram of body weight is the dosage which is on the packaging and/or insert that comes with whatever form you get.
I am not familiar with the liquid so I would check to see if it contains anything other than Ivermectin as an active ingredient. The paste is just Ivermectin, whatever the paste is and apple flavoring. (because horses love apple flavoring LOL)
Be sure you don’t get horse wormer that isn’t Ivermectin-based. The same companies offer a few types with similar packaging. Make sure it says Ivermectin.
IANAD - but I think either is OK, just get the dosing right.