This is very simple. Total up the price for the add-ons you require and add this to your pledge. Don't change the reward level though, as this will allow you to receive any of the stretch goals that you can get with your reward level and if you did change it, this may cause confusion and you not getting what you deserve.
If you pledge for Platoon level or higher (£60+) then you will receive your pledge level rewards as well as one of each of the stretch goals for the army which platoon you have selected. If you pledge for the Border Assault or Army tiers then you have pledged for both armies and will get all the stretch rewards.
You can only swap the stretch goals for an extra rifle squad. The Stretch goals are all of different values so this is not possible or easy to swap them around. All the items (as they become unlocked) will be available as add-ons for which you can add to your pledge total. This way, your Finnish army can have a T-26 tank or the BA-10 armoured car or the Russians could have a ski troop.
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