Reading the rules, activating a akill or tactic is an action.
So how would it work to activate them mid-combat?
Your Action is much more flexible than Combat or Movement, so depending on what you're using it on you can spend it during combat or sometimes even outside of your agent's turn (this is much more rare). However, you only have one Action to spend each turn, so use it wisely! It's hands down the thing that's going to be limiting you the most (which is mainly why some of us have come up with action point systems).
Also note that if the skill/tactic says it's a "reaction", then you don't have to use your Action to trigger it. If it says it is "passive" it's always active and doesn't require triggering explicitly at all.
Second question: Is it possible to activate both skill and tactic (e.g. +1 hit dice and Eagle Eye) for the same combat?
Usually not, but there are exceptions. If one of them is a Reaction, then it doesn't require the use of your Action. Sometimes skills are paid for with something other than Action (like the sniper's "sacrifice your movement to reroll failed dice" skill), and that can allow you to get fancy.
Third question: Are you allowed to activate a tactic like reroll ammo/jam/skull even after having used an action before combat?
You only have one Action, Movement, and Combat for the entire game round (regardless of when you use them), so if you spent your Action prior to your agent's turn, you cannot use it again until the next refresh phase.