1) He poisons his arrows (You can simply allow this with no roll, make a Common Knowledge roll with Smarts, or use an applicable skill such as Alchemy/Knowledge (Poisons)/Craft Weapon/Repair, etc.) If he rolls a 1 on his Shooting die or a critical failure, innocent bystander kicks in as he shoots an ally or critically shoots his own foot and he may end up poisoning an ally or himself instead. Note that death dealing poisons are very powerful and that the paralysis and sleep poisons are almost as bad (as it incapacitates a character and leaves them vulnerable to a finishing move on the battlefield) and are triggered on any damage result including Shaken (even a scratch will inject the poison).
2) Shaintar has the difference between a trained animal for combat and one for riding. The riding animal makes a Spirit roll -2 or runs away from combat situations whereas the war animal is used to battle situations. The war horse is trained and the riding horse is untrained.
As for paying for skills training, 50 Fathoms allows some trainers to improve the die type of a skill for 1000 pieces of eight for 7 days of training, which is what i would go with for a price/time (1000 money of your setting such as gold). Its really not worth it on an extra though as they can be killed relatively easily (and conversely very powerful on a much tougher wild card from familiar/sidekick/behold the pale horse).
As David said, Beast Master is the best way to handle this type of idea with the rules.
One way to do a whole lot of animals might be taking the Beast Master edge multiple times, once per animal companion desired with a limitation of taking the edge once per Rank (So it maxes out at 5 total animal companions with a cost of 5 advances versus the 5 followers for 1 advance, although followers aren't replaced). When one of the creatures are slain, a replacement is found in 2d6 days as well, so you don't interrupt play to replace them. The character would also benefit from Beast Bond and Bennie expansion edges like Luck/Great Luck to be able to give Bennies to animal companions for rerolls or to soak.