There was a video on the Youtube site referenced to on Stamp Community about using shellite to get modern stamps off paper and also using a steel rule to help scrape the gum residue off if I remember correctly.
Here it is after I searched for it:http://www.youtube.com/watch?v=nWMcVYgIZjM
Nice Tonga stamps! I like them, haven't seen them before.
I know I have some of those Sierra Leone map stamps on paper around here somewhere, hope they are not as difficult.