Hi Matthew...., I remember when I first started out, all my guys were built straight from the instructions, and altering even someone's hat scared me to death....
What I do now is go through the 'spare parts' of any old figures I've got, although everything is still 1/35 scale, arms, hands etc... can still be different sizes. Then maybe using clay or putty, alter the direction you want the arm to go in, shaving some plastic off if necessary, and filling the gap, sand off any excess & you should have a decent alteration.....? It's worked ok for me in the past.
There are some guys here who are real artists, can build a figure from scratch & you'd never know the difference..... Abit outta my league though..
Good luck.
Simon.